Since yesterday, when I try to post a message to one of the mailing list of my project (stellarium) from gmail, I get a 550 error. Exactly as described on your page:
Obviously gmail servers accept email on postmaster account (It works for other team members), so the problem must come from your side somehow. Especially the message "result of earlier verification reused" suggests that a failure from yesterday (gmail was down for some hours) created a real 550 error, but now your server cached this result and doesn't try again..
Manager of Stellarium