There are some situations where VPOP3 needs to know who is sending mail. This may be if you have VPOP3 configured to behave differently depending on who is sending messages - eg if you have different message size limits depending on the sender, or if you have message monitoring configured for certain users, or you don't want certain people to be able to send mail at all.
SMTP was originally designed to that anyone at all could send mail, but there were later extensions added to allow people to log on before sending mail. The default configuration of VPOP3 allows anyone on your local network to send mail without logging on, but this means that VPOP3 cannot always tell who is trying to send a message, to any restrictions may not work reliably.
In this case, you should enable SMTP authentication. To do this, go to Services → SMTP Server → General Tab and turn on Require SMTP Authentication. If you want incoming SMTP mail to work, you should also turn on Do not require SMTP authentication for incoming/internal mail (because external mail senders won't know login details)