Quote Originally Posted by bottyz View Post
After further probing the emails appear to be arriving ok to some email addresses and unformatted to others. For example, the emails arrive fine to my outlook account at work, and arrives wrong to my gmail.
Okay, so the stripping of content is happening by the email service *receiving* your emails.
That is presumably because they don't trust your content.
And that is often because you're sending it unauthenticated (PHP often sends email as user "nobody", which is the same technique spammers use, and thus often gets flagged as untrusted).
If you switch to using SMTPAUTH then your emails will be sent from an actual user of an authenticated email service, and the receiving system will be able to probe for MX validation when checking whether the content is trustworthy. (The concepts related to that are in the docs I linked you to earlier.) Thus the emails are more likely to show up in their full context.

Do keep in mind that webmail services DO alter your email content to suit their needs. Remember, in those cases the content is fitting inside existing <html> and <body> tags, so you need to follow the rules for appropriate formatting to suit. You can study HTML rules at the w3schools website.