Some ISPs (especially in Germany) require you to log on to collect your POP3 mail before they will allow you to send any mail through their SMTP relay servers.
VPOP3 actually connects to the POP3 server and SMTP server at the same time, so usually it will not have successfully logged onto the ISP's POP3 server before it tries to start sending mail.
You can tell VPOP3 to wait until it has finished retrieving mail before starting to send outgoing mail.
In the VPOP3 settings go to Utilities → Misc Settings → General in VPOP3 and turn on POP3 then SMTP Connection and save changes.
Also, go to External Mail → Connection Schedule and turn off If outgoing mail arrives whilst online, send immediately.