Ok. I have read through the various forum posts, the wiki articles and other things. I can't locate a situation that is similar so I am throwing my problem into the ring.
Issue: I am receiving the following message when trying to send mail through the web client only:
554 5.7.1 : Client host rejected: Access denied
This comes up on every recipient I try, no matter whether the domain is local or external.
I can send mail using Apple Mail, iPhone 4, and iPad, all of which are configured to use IMAPS with the SMTP server using SSL and password authentication on port 587.
Please note, that I also have to route all my outgoing mail through the Comcast MTA using SMTP Auth. I configured that as per the wiki article and it was all working fine, both webclient and non-webclient connections. Yesterday I found that my non-webclient would no longer send. I looked into this and changed a couple of settings as per the SMTP Auth Problems wiki article. Now I can't send from the webclient anymore. I undid the changes, which broke everything, then put them back so that I could at least send mail from my iPhone. Incoming mail works perfectly with no reall issues (excpet a silly issue with ntp, but that is not related to this).
I have attached the outputs of a zmprov gs command and a postconf command.
As for the DNS setup, I am running my own DNS server internally and forwarding all external DNS queries out to OpenDNS.