Four days ago I stopped getting mail from my academic mail account on my iPhone and Mac Mail. I was in a bit of a crunch so didn’t really pay attention since I still got some mail on my business and non-profit accounts in a unified Inbox. Imagine my surprise Monday when I realized I hadn’t received new and important e-mail since Thursday!
Apparently Gmail engineers fixed some bugs in the last month or two and when those changes propogated to my account, it changed the behavior of certain filters that were not, er, perfectly well defined. If you had filter (such as before:2011/01/01) to cause all messages before a certain date to skip the Inbox (which is useful when migrating from old accounts), then this may have happened to you. The Gmail Inbox feeds through Imap to the iPhone and Mac Mail clients, so they would no longer see new mail. The mail is still there, just under “All Mail” instead. Fix the filters, and life goes back to normal.