Unfortunately it is not addon.
For example, mail... If I buy the puri pack they have on sale today, I can get 5 or 7 of them and they drop into IS backpack instantly. If I send it as present to alt, it doesn't register. It supposedly sent it, but recipient does not get it. Then 5-6 minutes later "you got a present". There is no addon involved there, it is straight ROM. Or I have "Your item xxxxx has been sold" message from AH. Then again 5-6 minutes later "new mail" pops up.
Mail, which is either separate server or separate subsystem on same server, is clogged. As far as I know, messages are not getting lost, although some people claim they are. But I think all of those claims got it eventually, just quite a bit later, after they made the claim.
AH is similar, although just everyone uses AAH so addon is involved. But it is not addon acting up. Auctions are not listing without delay, searches failing, history of sales not popping up - all of it is ROM.
-- Rustyx --- 92R / 92S / 92M on Reni (Guild KnightShift). Yes, running the new FOTM R/M, cause I am not elf enough to be WD/S.
Oh, and people who have more than 3 classes are clinically insane.