I had the same problem last week after upgrading to Quickbooks for Mac 2020. Ultimately, it was Gmail blocking it. Apparently, they deem this latest upgrade "insecure". I had to login to my Google account and first setup 2-step verification, then request an app password. Then use that password in Apple mail instead of my usual one. Now it's working just like before the upgrade. Hopefully, this will help. Even if you use different applications, maybe the solution will be similar. Good luck. I should also add that in Quickbooks: Preferences: Email - I had to select "Custom" instead of "Default Email Application" and set it up there.
