Outlook problem in Mail

During the last three days I am having issues with my Outlook Exchange account that I use in the Mail app. I never had a problem before. I can send mails. I often do not receive the mails sent but once in a while they arrive. Connection Doctor does not give any error. It says connection to the server succeeded. I also have rebuilt the inbox. After the rebuild it received new mail arrived but then stopped again receiving mails. The issue happens both on my iMac but also on my MBP.