AOL is rejecting emails from my client's website, how can I troubleshoot?

A packet capture showing the SMTP conversation would be useful here, probably more so than the contents of your mail log. Also be advised that AOL's mail servers are pretty psycho about who they receive mail from. Check out their FAQ at http://postmaster.aol.com/guidelines/standards.html:

• AOL's servers will not accept connections from unsecured systems. These include open relays, open proxies, open routers, or any other system that has been determined to be available for unauthorized use.

• Ensure all forms on your web server are secure. If you use formmail scripts, ensure they cannot be used to send spam.

• AOL's mail servers will not accept connections from systems that use dynamically assigned or residential IP addresses.

• Organizations may not hard code AOL's MX records into their configuration files.

• All e-mail must be compliant with RFCs 2821 and 2822.

• AOL's mail servers will reject connections from any IP address that does not have reverse DNS (a PTR record). All e-mail servers connecting to AOL's mail servers must have valid and meaningful (not dynamic-looking) reverse DNS records. For example: Meaningful RDNS: mail.domain.com Generic RDNS: 1.2.3.4.domain.isp.com

• AOL may reject connections from servers whose recipient lists consistently generate a higher than 10% bounce failure rate. (i.e. over 10% of a sender's mailing list is destined for users that do not exist on our system)

• AOL may reject connections from senders who are unable to accept at least 90% of the bounce-return messages (mailer-daemon failure/error messages) destined for their systems.

• Complaints submitted by AOL members may be used as a basis for refusing connections from any mail server.

• Persons sending bulk mail from the whitelisted IP Address must immediately remove any e-mail address which causes a permanent failure "bounce" message to be generated with one of the following error codes: 550 "AOL username" Is Not Accepting Mail From This Sender 550 Mailbox not found 550 We would love to have gotten this email to [email protected]. But, your recipient never logged onto their free AIM Mail account. Please contact them and let them know that they're missing out on all the super features offered by AIM Mail. And by the way, they're also missing out on your email.

• AOL, LLC does not authorize the use of its proprietary computers and computer network to accept, transmit or distribute unsolicited bulk e-mail. AOL's unsolicited bulk e-mail policy can be found at our website.

I've bolded the points you may want to look into. I've had trouble with sending mail to them in the past, which turned out to be a misconfigured PTR record for the server that was sending the mail. I think you're OK there, but you should check the others out.

I imagine that you are not correctly setting the FROM header on the PHP side. It would help to post the PHP code that was used to send the email.

If you're doing email, you want to sign up for a feedback loop with AOL, and then to be on the AOL whitelist (which requires your server to be well behaved). It's worth it in the long run.

Post a capture of the SMTP conversation (anonymized as you like) so we can help more.