Please note: This is an unnofficial response (in other words I speak on behalf of myself, not LP):
The common reason lately is because they have had to disable to email service (checking only) all messages still are received, to allod the "RAID battery" to "recharge". I'm not exactly sure the reason for this problem, but I do know that there is a big announcement coming up soon about some hardware improvements that will solve this problem once and for all. Basically a global storage system.
Should be announced in the next 2 weeks I hope! Once this is rolled out, the RAID battery issue will be history, and there will be no need for any periodic delays in receipt of email.