Problem with Apple Mail on High Sierra

Hi,

I have a Mid 2010 Mac Pro with High Sierra installed. Since a few days Apple Mail gives problems. Anytime I send an email it opens the window which appears when a server is unavailable. However, the email was already successfully sent and the email with the problem window is entirely empty. It is a huge annoyance in my workflow. The problem I get is that the receipient is not specified.

Can anybody help? I already deactivated an reactivated all email accounts and reindexed them...

Cheers!