This is a Web mail setup for about 50K clients. Initially we require your proposal and capacity planning. here i can share some information, 1. You may use Postfix, Cyrus IMAP or Zimbra collaboration server, Nginx IMAP/POP3 proxy, Apache, Memcache & Mysql etc… (Budget: $1500-$3000 USD, Jobs: Apache, Linux, MySQL, Nginx, System Admin)
