The Apple Mail application on my wife's imac is set to IMAP and her iphone is set to POP for the same Comcast email account. She discovered that her inbox on the imac (IMAP) only goes back to mid-November of last year. It should go back a lot further. She doesn't use the phone (POP) actively for email, but looks at it sometimes. I haven't opened mail on the iphone (POP) but based on the number overlaying the icon I am guessing the iphone has a lot more mail on it than the imac.
I would like to switch the iphone to IMAP, but want to ensure that if there is a larger inbox on the iphone it is retained and is visible (and synced) on the imac.
Suggestion-------------on the phone, recreate the account from scratch but let Mail auto-configure it for you. Comcast accounts are IMAP compliant by default, and the phone will detect that and set it up accordingly. The account should now be synced on the phone and the Mac. Once you are satisfied everything is OK, then you can go back on the phone and delete the POP version of the account. Using different protocols on two devices for the same account is a recipe for trouble.