Slow and peculiar behaviour of Mail Version 13.4 (3608.120.23.2.1)

It seems as though your email is not being sent, and the reason is probably an authentication failure. The first screenshot is unsurprising since that result will occur with any failure to authenticate, even if you are completely certain the password is correct.

The usual fix is to remove the affected Mail account, and add it again. Delete the account from System Preferences > Internet Accounts. Then, add it again using that same Preference pane. Those instructions are provided in Set up Mail with email accounts - Apple Support.

Does Mail have a limit to amount of Mail boxes or accounts?

None that I am aware of. Whatever the problem is I doubt it's that.

I have tried removing and re-installing the ymail account which I mostly use and it has not worked. Send failure recurs. I also had to do a fresh install of Catalina because of a concern about a security issue. It was not long before the mailbox problem again intermittently occurred. Apple must have a better response, surely, or maybe they are whooping it up with their 2 trillion capitalisation. Some resources targeted on this problem would be really nice, even if it's a shared problem with the mail provider.VERY frustrating.

HI< Am having the same issue on my Mac. On webmail iphone and macbook, its working fine.