If you see the text 'MX Send Warning - <error message>' in the VPOP3 status monitor, that means that VPOP3 is trying to send a message to another mail server using the MX send method (direct to the recipient's mail server) and the other mail server is giving the <error message> error.
If the <error message> begins with the digit '4', then VPOP3 will automatically retry the message later.
There is nothing you can do about this because the error is coming from the recipient's mail server, not from VPOP3. If you have to do something, then you should contact the recipient using another method (eg telephone, fax) and tell them about the problem on their mail server.
This message will not be reported to the VPOP3 administrator because it is not a problem in VPOP3. If you use the MX sending method, then you should expect lots of these messages, and it is usually safe to ignore them. If VPOP3 cannot send the message to the recipient after some time (default 3 days), then it will send a delivery failure report to the original message sender who can decide what to do - eg confirm the email address, contact the recipient some other way, etc.