Analysis: Typical POP3
If any messages are being retained
- Re-lock mailbox
- If mailbox not empty
- Append new messages to temp file
- Truncate mailbox
- Merge retained temp file contents onto mailbox
- Unlock mailbox
Notes:
Does this sound like a lot of operations to you? Well, many of those operations are synchronous meta-data operations, and therefore exceedingly expensive to incur on a mail server.