Email message not correctly formatted with PHP and outlook

I need to know if there is a way to format a text in the message body of an email in outlook.

I have this part of the message I have to send with mail function towards outlook:

Δ request and Δ fault must be....

Well, when php script sends email, in outlook I have this formatting:

Î" request and Î" fault must be...

How should I do to get the greek letter Δ correctly?