Spamdrain filtering supports two type of connections: IMAP and POP3.

POP3 is an internet protocol to retrieve email. Email messages are delivered and stored on your personal computer, mobile device or any other tablet or phone. When you check e-mail, read it, it is downloaded it to your computer and immediately deleted from the server. This means that read e-mail on the computer will not be accessible on the phone.
IMAP on the other hand saves and stores all your e-mail messages on the server. The advantage of IMAP access is that you can check and send e-mails on multiple devices because you always interact with a mail server and not just one device. IMAP protocol is better and more modern than POP3 protocol. Using IMAP connection allows you to store emails on the server and always have access to them (with the internet connection of course ). There is also an advantage of POP3 connection: you can read e-mails even when you are offline.