Our IMAP/POP servers on the new machine
aqua-new.rahul.net were sometimes accepting insecure (un-encrypted) connections on ports that were supposed to accept only secure (encrypted) connections.
In about two days from now, the configuration will be made more strict. Insecure connections will be accepted on ports only where the documentation says so. The purpose is to make sure that if you are using an insecure connection, it is a deliberate choice and not an accident.
The page below lists which types of connections will be accepted on which ports.
Classic Linux ports and protocols
It is possible that you were using an insecure connection without realizing it. If so, then after the reconfiguration you might have difficulty connecting. Just take a look at the documentation page above and make a minor adjustment as needed. (In the mean time you can always use Roundcube webmail to access your email.)