On: Why does Gmail hate my domain?

[CDokolas]

Sounds like re-training Google's spam filter with a few false negatives (by marking them as such). Am I right?

[kazinator → CDokolas]

Google mail's spam filter delivers spams to users; it just classifies it into a different folder.

In this case, the mail was bounced at the SMTP level, and the bounce is (strangely) identified as coming from Google Groups.

"Mark as safe" will not SMTP non-delivery problems, I think, just the mis-classification of e-mail as spam at the folder delivery level.

[ciaoben → CDokolas]

exactly. That is spam detection programs work. If you re-train them, when they think that the volume of spam sent from that domain in acceptable and normal, they will let its email pass.