Email body arrives as attachment on microsoft exchange server

[yisraelharris]

I am using a standard smtp mail function, written in php, to send a text/html email:

($fp = fsockopen($host, 25, $errno, $errstr, 60); etc. etc.)

Tested on many recipients, and it works fine, with one exception: for one recipient, the entire content of the email arrives as an attachment!

After a bit of sleuthing, I learned that all other recipients are pop3 email accounts, while the problematic account is ""microsoft exchange server". Any ideas how to solve the problem? Please note that when I send email to his recipient with the standard php mail() function, it works fine.