Do you check your email on more than one device?
Maybe you use a desktop at the office and a laptop or tablet at home? How about on your phone so you can get email wherever you are?
This is NOT a technical post, it’s a “practical” one that can save you a whole lot of trouble and it’s SO very easy to understand and follow the suggestion.
Whenever you setup ANY email software you MUST be very sure that any and all devices used to check a specific email account are either POP or iMap! If you mix these email protocols you are asking for trouble like lost email and mail that appears on one device but not any others. This can be a huge pain in the neck for you so just assure ALL devices are set to use the same protocol for each email account.