Are your senders with comcast.net? I have had this issue with one sender, and it seems others have also based on other posts, but everything points to only the relationship with comcast. Not all of the messages were being delayed either.
Based on previous discussions with Apple, the me.com email address will be supported indefinitely. The only thing that is changing for users that were on me.com is the other features related I.e file storage through me.com.