I have the lovely task of importing a POP3 mailbox with 144524 messages into our database. I'm using a 3rd party component, but am quickly learning more than I ever wanted to know about POP3. For example, ideally you don't want to use POP3 for large mail boxes because POP3 isn't scalable.
Having read RFC 1939 which specifies the Post Office Protocol Version 3, I now understand why it isn't scalable. There are three phases to a POP3 session, the AUTHORIZATION phase, the TRANSACTION phase, and the UPDATE phase.
The AUTHORIZATION phase is simply a login phase. The USER command specifies the username to log in the mailbox with and the PASS command specifies a password. Once authenticated, the server enters the next phase.
The TRANSACTION phase is where the POP3 client does the real work. For example, the STAT command returns the message count within the mailbox. In order to retrieve an individual email, the Message Id is required. To get that, issue the LIST command with no arguments, and the POP3 server will list all the message ids and the size in octets. There's no way to specify a number of messages to return. So in my case, the command has to return the IDs and sizes for all 144524 messages. But wait, it gets better.
Once, a POP3 client is done issuing commands to delete messages in the TRANSACTION phase, none of those messages are actually deleted until the client issues the QUIT command. At that point, the POP3 session enters the UPDATE phase and the server starts to delete messages marked for deletion. As you can deduce, that could be problematic for a large mailbox.
Also, you might consider deleting messages in batches which is fine, but there's a hitch. The QUIT command terminates your session after the UPDATE phase is over and POP3 does not guarantee that the message ids you listed before will be the same in a separate session. Therefore you have to issue the LIST command at the beginning of each session to list every message even if you only plan to process a small subset.
In any case, I'm working with the author of the POP3 component I'm using to iron out some kinks and make this work. Once I trim this mailbox down, it should hopefully never get so big again.