Trying to dissect that error message:
Diagnostic-Code: smtp; 550-Verification failed for
<firstname.lastname@example.org> 550-The mail server could not deliver mail
. The account or domain may not exist,
they may be blacklisted, or missing the proper dns entries. 550 Sender
I know that the domain exists, I can ping it, and I can browse it, and I can dig it.
How do I tell if that domain has been blacklisted, and more importantly, how do I change that? If this is a dns issue, then I can only guess that it is a reverse DNS lookup that may be failing, since I can ping/browse/dig by name, but how do I check and/or fix that too?