Sometimes people need to copy outgoing mail for various reasons. There are several ways of doing this in VPOP3 depending on your exact requirements:
For most people, the first three of these options are often the most suitable.
Message Monitoring is often most useful if the person who is monitoring the message needs to know that a person has sent a message, and needs to check it, for instance, if the person being monitored is a trainee, or is not trusted.
Message Archiving is often most useful if you just need a copy of messages for tracking or dispute resolution purposes. During normal operation, no one will know that the messages are being archived, but if you need to look back at messages sent or received by someone, then you can search for them.
Shared IMAP4 folders is most useful if the viewing of outgoing mail is agreed with the sender, and is just used for convenience's sake, rather than for disciplinary, dispute or legal reasons.
You can use any combination of these methods, as you require.