WordPress MU

It appears that users on 'comcast.net' email addresses (and possibly some other domains) are not receiving registration or forgot password emails.

We're running WPMU on a dedicated OS X server with a static IP. WPMU is sending mail using php mail() so not surprisingly /var/log/mail/ is empty!

It's really strange because if I send personal email to 'comcast.net' from the same domain name (routed via Gmail Google Applications) it works perfectly fine.

You just said it - the one routed thru Gmail comes thru as legit. Because Comcast recognizes gmail.

I ran up against the same issues, same provider. Do a reverse DNS on your box. Google will help there. ;)

I set an additional MX record for the WPMU server and it did not help!

WPMU sends emails with 'fake' from field - wordpress@yourdomain.com
If you don't have such email box on your server - most mail servers won't accept messages.

Comcast is famous for blocking IP addresses for spam.
I was assigned a brand new IP (direct from ARIN) it was blocked by Comcast. It took close to 6 mos for Comcast to correct this issue. Even though when we ran made all corrections on their website.

Until this problem was corrected NO ONE on my network could use the email from that IP.

You might want to check that out to see if it is a problem for you also.

Its probably not a blocked/banned IP because we have Google Applications handling our individual mail accounts for the same domain. If I send mail from my personal account (through the gmail client) it works fine to Comcast. If I send it through a simple php mail() script it disappears.

Its not just Comcast we've noticed other (less popular) domains are having the same problem receiving mail from our server.

Try signing up at your WPMU using some of your emails and if the message arrives - show me full headers.