Known problems of the original Microsoft SBS POP3 Connector, the one that is included with the Small Business Server:

If there is a malformed header line the e-mail, the user doesn't get e-mail. After some time the POP3 connector stop processing the mailbox. The fix is to manually delete all messages and recreate the mailbox.

The POP3 connector can't download an e-mail over 10MB even your Exchange has no limit, becuse the limit of 10MB is inside the POP3 connector.

Small Business Server SBS 2003:

The POP3 connector can't collect faster then every 15 minutes.

The POP3 connector retrieves the user's e-mail (john.doe@domain.com) and routs it to the appropriate local Exchange mailbox (jdoe@domain.local), just like it's supposed to do, but only if jdoe@domain.local is set as the primary reply-to addresses under the E-Mail Addresses tab in the user's properties.

The POP3 connector bypasses spam and virus checkers.

The POP3 connector can't handle BCC, even when the ISP adds them to the header lines.