Getting some unnecessary and unwanted code in Gmail content email by html and php

Attaching snap of what other person is getting if are sending mail:
enter image description here

Here is the html of that email – I request you to please help me identify the issue behind it. may be i am not providing you the full info or something else for that i am apologising in advance. I will be very thankful for your suggestions.

using this html:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html>

<head>
    <META http-equiv="Content-Type" content="text/html; charset=utf-8">
    <style>
        body {
            margin: 0;
            background-color: #E1E2E3;
        }

        p,
        ul,
        li,
        strong {
            color: #000 !important;
        }

        a {
            color: #1C509D !important;
        }

        table {
            border-spacing: 0;
        }

        td {
            padding: 0;
        }

        .main {
            background-color: #FEF1E8;
            max-width: 670px;
            margin: 0 auto;
            width: 100%;
            border-spacing: 0;
        }

        .para-content {
            font-family: 'Arial', sans-serif;
            font-style: normal;
            font-weight: normal;
            font-size: 14px;
            line-height: 19.6px;
            margin-top: 0;
            margin-bottom: 0;
            padding-top: 10px;
            color: rgba(0, 0, 0, 0.6);
            mix-blend-mode: normal;
        }

        hr {
            border: 0;
            border-top: 1px dashed rgba(0, 0, 0, 0.25);
            margin: 0;
        }

        .small {
            font-family: 'Arial', sans-serif;
            font-style: normal;
            font-weight: normal;
            font-size: 14px;
            line-height: 19.6px;
            margin-top: 0;
            margin-bottom: 0;
            color: rgba(0, 0, 0, 0.6);
            mix-blend-mode: normal;
        }

        .heading-small {
            font-family: 'Tahoma';
            font-style: normal;
            font-weight: 400;
            font-size: 20px;
            line-height: 120%;
            margin: 30px 0 0;
        }

        .text {
            text-decoration: none;
            padding: 5px;
            color: #1C509D;
        }

        .border {
            border: 0;
            border-top: 3px solid #000000;
            margin-top: 0;
            margin-bottom: 0;
        }

        .table {
            text-align: center;
            padding-top: 50px;
        }

        .body-text {
            font-family: 'Georgia';
            margin: 10px 0;
            font-size: 16px;
            line-height: 150%;
            color: #000 !important;
        }

        .main-date {
            font-family: 'Arial', sans-serif;
            font-style: normal;
            font-weight: normal;
            font-size: 14px;
            line-height: 19.6px;
            margin-top: 0;
            margin-bottom: 0;
            color: rgba(0, 0, 0, 0.6);
            mix-blend-mode: normal;
            padding-top: 50px;
            text-align: right;
        }

        .logo {
            padding-top: 3.125rem;
        }

        .button {
            margin-top: 24px;
            font-family: 'Arial', sans-serif;
            font-weight: 700;
            background-color: #1C509D;
            border: none;
            color: white;
            padding: 10px 15px;
            text-align: center;
            text-decoration: none;
            display: inline-block;
            font-size: 14px;
        }

        @media (max-width:768px) {
            .main {
                padding: 0 15px;
            }

            .main-mob {
                padding: 0;
                margin-bottom: 15px !important;
            }

            .mob-dots {
                padding-left: 15px !important;
            }

            .main tr,
            .main td {
                margin-left: auto;
                margin-right: auto;
                display: block;
            }

            .logo {
                padding-top: 1.875rem !important;
            }

            .small {
                text-align: left;
            }

            .table {
                padding-top: 30px;
            }

            .main-date {
                padding-top: 15px !important;
                text-align: left !important;
            }

            .button {
                margin-top: 20px !important;
            }

            table {
                width: 100%;
            }

            .p-mob-30 {
                padding-top: 30px !important;
                padding-bottom: 30px !important;
            }

            .main-logo {
                width: 270px;
            }

            .logo-align {
                text-align: center !important;
            }
        }
    </style>
</head>

<body>
    <center>
        <table border="0" cellpadding="0" cellspacing="0" width="670"
            style=" border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%; border: 1px solid #DDDDDD;border-radius: 4px;">
            <tbody>
                <tr>
                    <td
                        style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                        <table align="center" border="0" cellpadding="0" cellspacing="0" class="main" width="670"
                            style="background-color: #FEF1E8;border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                            <tr>
                                <td
                                    style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                    <table width="50"
                                        style="border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                        <tr>
                                            <td width="100%"></td>
                                        </tr>
                                    </table>
                                </td>
                                <td
                                    style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                    <table border="0" cellpadding="0" cellspacing="0" align="center" width="570"
                                        style="background-color: #FEF1E8;width:100%;border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                        <tr>
                                            <td style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;"
                                                class="logo-align">
                                                <a href="#" class="logo" style="display:block;padding-top:40px"><img
                                                        src="https://www.demoonline.com/assets/images/demoonline-mobile.png"
                                                        width="330" alt="" class="main-logo"></a>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td colspan="2"
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <div class="p-mob-30" style="padding-top:30px;padding-bottom: 30px;">
                                                    <hr class="border"
                                                        style="border:0; border-top:3px solid #000000;margin-top:0;margin-bottom:0;">
                                                </div>
                                            </td>
                                        </tr>
                                    </table>
                                    <table align="center" border="0" cellpadding="0" cellspacing="0"
                                        class="main main-mob" width="670"
                                        style="background-color: #FEF1E8;border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;margin-bottom: 60px; font-size: 16px; font-family:Georgia, 'Times New Roman', Times, serif; font-weight: normal;">
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p style="margin-top: 0;" class="body-text">Dear
                                                    {contactfield=fullname|Subscriber},
                                                </p>
                                                <p style="margin-bottom: 0;margin-top: 20px" class="body-text">The April
                                                    2023 issue of Mutual Fund Insight has been published
                                                </p>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p class="body-text">Here's what we are covering-
                                                </p>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p class="body-text"><strong>Cover Story - </strong><strong>India’s
                                                        great growth story: Invest in it, grow with it.</strong>
                                                </p>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p class="body-text">Did you miss the bus when we splashed 'Equity
                                                    Without Fear' on our magazine cover 20 years back?
                                                </p>
                                                <p class="body-text">Don't feel bad, 90%+ Indians failed to capitalise
                                                    on it.
                                                </p>
                                                <p class="body-text">The good news is that there's still time. With an
                                                    unprecedented transformation
                                                    in the history of the country, India's financial markets are giving
                                                    you a second chance to right your wrongs.
                                                </p>
                                                <p class="body-text">In this month's cover story, we tell you how and
                                                    why India has hit the highway to growth -
                                                    and will continue to remain there. (Some of the numbers in the story
                                                    will definitely wow you!)
                                                </p>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p class="body-text"><strong>Other Stories:</strong>
                                                </p>
                                                <ul style="padding-left: 1.5rem;">
                                                    <li style="margin-bottom: 0.5rem;">Pessimists sound smart but
                                                        optimists make money: First Page column by Dhirendra Kumar</li>
                                                    <li style="margin-bottom: 0.5rem;">A 2023 roadmap for international
                                                        investors</li>
                                                    <li style="margin-bottom: 0.5rem;">Municipal bonds: The new kid on
                                                        the block</li>
                                                    <li style="margin-bottom: 0.5rem;">Knocking on US bonds’ door</li>
                                                    <li style="margin-bottom: 0.5rem;">The curious case of Quant</li>
                                                    <li style="margin-bottom: 0.5rem;">The Adani saga exposes a glitch
                                                        in passive funds</li>
                                                    <li style="margin-bottom: 0.5rem;">Spotlight I Interview with Rahul
                                                        Singh- Chief Investment Officer - Equities, Tata Asset
                                                        Management</li>
                                                </ul>
                                                <p class="body-text">And like every month, we are carrying special
                                                    columns, newsmakers, answers to your saving and investment-related
                                                    queries, the most comprehensive fund scorecard, and more.</p>
                                                <p class="body-text">Now available in print and digital format.</p>
                                                <p class="body-text"><a
                                                        href="https://www.demoonline.com/store/emag/mfi/2023-04/?utm_source=email_mfi_apr_subs&utm_medium=subscriber_email&utm_campaign=mfi_aprissue_announcement">Click
                                                        here to download</a>
                                                    your April 2023 Mutual Fund Insight issue.</p>
                                                <p class="body-text">Happy Reading!</p>
                                            </td>
                                        </tr>
                                        <tr>
                                            <td
                                                style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                                <p class="body-text">Regards,<br />Subscription Team</p>
                                            </td>
                                        </tr>
                                    </table>
                                </td>
                                <td
                                    style="mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                    <table width="50"
                                        style=" border-collapse: separate;mso-table-lspace: 0;mso-table-rspace: 0;-ms-text-size-adjust: 100%;-webkit-text-size-adjust: 100%;">
                                        <tr>
                                            <td width="100%"></td>
                                        </tr>
                                    </table>
                                </td>
                            </tr>
                        </table>
                    </td>
                </tr>
            </tbody>
        </table>
    </center>

</body>

</html>

I tried to find any reason by analysing html, I want to confirm if it from html side or is it generation from php side?