If you get this error message from within Webmail, then it usually means that the SMTP service access restrictions are blocking Webmail from sending mail.
The VPOP3 Webmail service sends mail by sending it to the SMTP service on the address 127.0.0.1. It can bypass authentication requirements, but if you have set the SMTP service not to allow connections or relaying from the 127.0.0.1 address, then you will get an error such as the above.
To fix this, in the VPOP3 settings go to
Services → SMTP Server. Go to the
IP Access Restrictions tab and Add a restriction rule like either Allow | Local Nets or Allow | IPv4 | 127.0.0.0 / 8. Then press the