I recently started getting complaints from several users that their friends were getting bounce messages when trying to send them email. When a user was finally able to produce the actual bounce message for me, the problem seemed to be with rhsbl.sorbs.net.
My sme server has been configured to use
rhsbl.sorbs.net for RHSBL service for over a year, but has only recently started blocking all emails from gmail.com, aol.com, hotmail.com, yahoo.com, juno.com and others (or my users have only recently decided to mention it to me).
(Note: I checked a couple of these domains on the sorbs servers at
http://www.sorbs.net/lookup.shtml, and they've been listed for a while. I don't know what this indicates, if anything).
For now, I have reconfigured my SME server as follows:
config setprop qpsmtpd SBLList badconf.rhsbl.sorbs.net:nomail.rhsbl.sorbs.net
signal-event email-update
Edit: made this post sticky for now