Should I use SSL for POP/IMAP/SMTP?

You should *always* use SSL. This will help prevent bad actors from intercepting your connection...

 What's the difference between IMAP and POP?

IMAP works with email directly on the server, while POP fetches mail from the server and stores...

 What's the maximum attachment size?

The maximum size for an attachment is 50 megabytes. Message that exceed this limit will be rejected.