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?
Update
As I no longer work at that company I'll have no opportunity to test any new ideas. If you have any ideas about this problem then by all means post them for the benefit of others who may experience a similar issue but I'll be unable to accept any answer on this question.
Best Answer
Outlook is very forgiving of formatting errors, and has been very good at introducing them. Try delivering them to a store where you can look at them in raw format. A local mailbox or Maildir store on the server would be ideal.
ContentHeaders may be checked by both desktop clients and webmail clients. Don't assume because it is webmail it supports HTML. Several webmail applications default to text mode due to security issues with HTML.
Ideally you should generate a multi-part MIME with both text and HTML format. This is the most flexible option. Properly done it should display well everywhere.