In any case, it's NOT Anope's fault nor can we help you further. Anope doesn't pick and choose who gets mailed and who doesn't. As far as Anope is concerned .. it works or it doesn't, there is no "kinda". By your own admission Anope has sent mail before, that means Anope is working correctly.
Anope only send the mail to the specified server, beyond that it's totally up to all MTA's between yours and your users as to if it gets delivered.