@meyerske:
As mentioned by 'someone' IMAP is the most common protocol in the mobile world - for various reasons.
IMAP is designed with the goal of permitting complete management of an email box (your webmail account) by multiple email clients, Therefore, clients generally leave messages on the server until the user explicitly deletes them.
The IMAP protocol specifically allows simultaneous access by multiple clients and provides mechanisms for clients to detect changes made to the mailbox by other, concurrently connected, clients.
The (slightly) disadvantage of IMAP is that the definitions of how it works varies considerably between servers and email clients. For example, email clients using IMAP do NOT usually download and keep ALL your emails. Instead, an IMAP server is meant to be the holder of your emails. Your IMAP email clients only synchronize a configured number of emails, not generally everything.
Since your account is currently set up as POP3, Aquamail cannot convert this account into a IMAP account.
So, you'd have to create a new account in Aquamail - as a "IMAP account" for the same mail account, and then, when it is working, delete the old instance (in Aquamail).
Kind regards
Mikeone