Our company has one web site with an online store based on osCommerce. The system sends emails for various reasons, such as password changes, order confirmations, etc., using PHP's mail() function.

We occasionally have customers report that the email they received is either blank (email is plain text format) or gibberish (email is in HTML format). In the latter case it's really just HTML that's being displayed as raw text but of course the customers can't read it. In this case the first opening tag's <, and sometimes a few more characters, has gone missing.

In an attempt to determine whether this was happening only for certain customers or email systems I configured the web site to send a CC of each message to a service account at my end. Those CC'd messages always arrive intact and display correctly in Outlook. For what it's worth, it seems to happen a little more frequently to Hotmail users but is certainly not limited to them.

As the web site is on a shared (Debian) host there's precious little I can do about debugging things from that end, although if I made the right request I feel the hosting company staff would help me, even though they have limited resources to spend on such matters.

Any suggestions on what else I might do to try and determine just why those emails are not being received correctly by some customers, yet a CC copy arrives just fine?