Update: The issue appears to have been quietly resolved as of March 18, with Gmail to iCloud emails taking only a minute. Noted in reader tips and confirmed by 9to5Mac, the issue occurs when a message is sent from a Gmail account to an @iCloud.com or @me.com address, resulting in an extended delivery delay and/or this failure message from Apple’s servers:
451 4.5.3 Too many rejections; try again later.
According to the discussion forum threats, Apple’s servers have been rejecting Gmail messages since last week, and Apple engineers are continuing to work on a fix for the problem:
They are very aware of the issue and working on fixing it. They said that everyone who is having this issue should call and report this. They’ll ask you for some info that will help them. They [sic] tech said specifically that the more people that call, the easier it will be to get a fix.
If you’ve been waiting on important Gmail messages and haven’t received them, this is most likely the reason. Apple’s telephone number for reporting iCloud mail issues is 800-275-2273.