New mail remains in mailbox out

A mail wil not be send. It is waiting in mailbox out, but never leave it.
Receiving mails is no problem.
How to resolve?

As a test, can you completely disable any anti-virus, firewall, proxy or VPN, then try send again.
Sometimes even restarting your router may resolve this.

I did what you advised, but it was not succesfull.
Another idea what to do?