SOLUTION THAT WORKED FOR ME:
After many tries, I realized that the problem was gmail account.
So I deactivated gmail account from mail app and I went to gmail website and from the preferences I chose to forward emails to my apple mail.
After that, no more mail problems.
After many tries, I realized that the problem was gmail account.
So I deactivated gmail account from mail app and I went to gmail website and from the preferences I chose to forward emails to my apple mail.
After that, no more mail problems.