I have several POP3 e-mail accounts that periodically just "disappear" from my Samsung Galaxy S3. My enterprise accounts never disappears. I add the POP3 back and they are they for a while, then when I try to open a notification I get a message that says
MAIL NO LONGER RESPONDING and my accounts are gone.
I have tried deleting all the accounts, clearing the cache then re-adding, renaming the default name -- all to no avail. I am on AT&T if that makes a difference.