Mail app not sending mail through Exchange

How can I fix Mail.app not sending and neither receiving emails? I click "send" and they're stuck in the outbox. When I restart the Mail.app. the emails are sent and I receive emails briefly and then again nothing works.

In contrast, the problem does not occur in Spark email client on the same computers, neither on Gmail. Only the Exchange and only Apples Mail fails.

The problem occurs on both my iMac and MBP bought in Dec. 2019.

Please help. Thanks