I am the administrator of a Unix server that sends e-mails from home builders to real estate agents about certain buying incentives, new properties, and other news. Recently, I've noticed that not all of the e-mails are going through, or arriving on recipient's inboxes as HTML code. and I'd like someone with Unix, E-mail, and PHP experience to help sort out why.

Specifically, I need help understanding how to send Text and HTML e-mails with mime-boundaries and configuring/optimizing my Unix server for e-mails.