“Can’t send mail - the sender address was invalid.”
Whether creating a new e-mail or replying to one from my inbox, the same error occurs.
iPhone 8, iOS 16
Your email account is not set up properly. Go to Settings/Mail/Accounts and click on the account where this problem occurs. If it says that the account is IMAP at the top, just delete the account. Then restart your phone, and go back to the same Settings/Mail/Accounts and add it back.