When sending mail you may get an error from the remote SMTP server saying something like 'Invalid HELO name'
This happens if the host name configured in VPOP3 does not look like a valid Internet host name (e.g. 'mail.domain.com'). Some SMTP servers don't bother checking this value because it has no authentication value at all, but a few do check that it looks correct.
To correct this, in the VPOP3 settings go to Settings → Misc Settings → General and correct the VPOP3 Host Name value.
See the reference guide for more details.