Dear Nicole Feldman
Thanks for using Microsoft products and posting in the community.
We have found that many people are experiencing the same problem. We are now gathering information about it as well.
Now we need to make sure that the antivirus software installed on the computer is not causing the interference because the port of the new Outlook is different from the port of the classic Outlook, which is the same as the port of the web-based version of Outlook, so we may need to troubleshoot if it's the three-party antivirus software that's causing the interference (if there is one).
You can try checking the settings of the three-party antivirus program to see if there is a section about email, but of course, it's best to uninstall the antivirus program first.
Also, if other users are experiencing the same problem, I would like users to provide information about the system version, the domain where the mailbox is configured, and the domain where the problem is occurring, which will help with troubleshooting.
I hope the above information can help you.
Looking forward to your reply.
Best wishes
Clark | Microsoft Community Support Specialist