This SMTP error message means that the computer which is sending the message is not permitted to send outgoing messages based on the VPOP3 SMTP Service's Access Restrictions.

This is probably because someone is trying to send messages from outside your local network, and you have not configured VPOP3 to allow this. You have to be careful when allowing this, because you may configure VPOP3 to be an open-relay, which is a bad thing.

We have instructions here telling you how to configure VPOP3 to allow relaying of mail from outside your local network.

If you are having this problem from people inside your local network (who are connecting using the mail server's internal IP address) then it may be because someone has changed the default restrictions, or you may have a complex network with multiple subnets defined. In this case, you need to go to Services → SMTP Server → IP Access Restrictions, and check that the required subnets are allowed, or add a new rule to allow the required subnets.

IMPORTANT: Do not add a rule to allow 'any host' to send mail unless you are requiring authentication - see the link above for configuring relaying of mail from outside your local network.