So I'm using ActiveSync on my iPhone on the latest iOS, I'm able to receive emails, move them to folders but when i try to compose or reply an email, i will get an error "Cannot Send Mail. The message was rejected by the server."

I have 2FA enabled, I'm using the app specific password.

Reminder, I'm able to receive but not send. 

Tested via imap or zoho mail app, no issue. Only on ActiveSync.