Oh, I found it. One thing, both POP and IMAP are enabled. Could that be the culprit? Or, does it need both because the mail on the web hosting account is the POP where the incoming mail is retrieved from?
POP and IMAP are different methods of downloading mail to your client for reading. POP allows the client to delete old messages after they've been downloaded or deleted from your client, but IMAP has a lot more advanced features for synchronizing mailboxes.
Both POP and IMAP use SMTP to send mail.
One other major advantage of IMAP is the ability for several computers to be using the same mailbox at the same time, without losing mail. Changes made on one client get synced to all other clients. This also allows for reading your mail via webmail. But it also means that your mailserver will probably require more storage space since messages are being cached there.
So IMAP / POP are a bit of a tradeoff. Nowadays though with storage so cheap, IMAP is taking over, and will probably replace POP entirely in the near future. Neither should affect your mail client speed much. Though if you have a lot of mail in your IMAP account, you'll see a little lag when it has to sync changes. Receiving one or two new messages shouldn't have a noticeable impact. But this is comparing a change made on the client, not the server.
