What happens to my in-game mail when my World of Warcraft account is frozen?
Solution 1:
I have a terrible habit of not checking my mail before freezing my account, and I can confirm the clock stops for no man: after 30 days, the mail is returned, and after 30 more days, it's deleted.
Besides the clock, the mailbox works as if you were still subscribed. Your characters, for all intents and purposes, are considered merely "logged off" until you explicitly delete them, so mail will still be delivered even while your account is frozen.