Apple Mail stopped sending Mails via Exchange Server

I am using Mac OS 11.6 with Apple Mail 14.0 and German language.

A couple of days ago, Apple Mail stopped sending E-Mails via my Office Microsoft Exchange Server.

I've tried a number of workarounds, e.g. deleting the account, to no avail.

Whenever I try to attach a file, the mail winds up in the folder "Ausgang" (no idea what it's named in the US version. After some time span between 15 seconds and some minutes, a window opens and tells me, the mail could not be sent as the connection to the Exchange server could not be established.

Some times (not too often), it is sufficient to click "use selected server" (mit ausgewähltem Server versuchen).

In most cases I see the status "sending E-Mail" (E-Mail senden) but nothing else happens for up to two hours, the progress bar simply refuses to move.

Checking the connection to the server ("Fenster/Verbindung prüfen") does not give any clues, if takes ages but eventually claims that the connection was successful. This does however not result in a sent e-mail.

Receiving and sending mails to / from the same server works flawless in iOS.

Has anybody encountered the same issues? Solved the problem even?