Important to know the difference between IMAP and POP3: With imap there is a 2 way exchange of information, messages and their status. I.e. if you have your mail on a pc and a smartphone, and then delete a message on the smartphone, the message will be gone on your pc as well. Equally, if you send a message from one unit, it will show up in the sent folder on all units. As the smartphone fills up quickly, you will want to delete messages without loosing them on your office computer, which means that imap is no good without having to spend time on managing messages and archives before you can delete anything.
With the older pop3 on the other hand, you only fetch new messages to any and all of your units, and can delete on one unit without affecting the others.
Then there is server space to consider: With imap, everything is stored on both the server (isp) and your units, if you don't mess with archiving. With pop3, you set up one of your connected units to delete messages on the server after a certain time, e.g. 1 month, so that you can be sure of getting them to all your units, and also delete from server before your alotted storage is filled up.