The error message is “A copy has been placed in your outbox. The recipient “email@example.com” was rejected by the server.
If you receive the following error message above on your Apple Mail app.
Please follow the steps below:
Go to Settings --> Mail, Contacts, Calendars.
Tap on your Mailbox Name on the right side.
Now tap once again on your mail box email address
Under: OUTGOING MAIL SERVER:
Tap on the SMTP server.domain.com >
Under: PRIMARY SERVER:
Tap on: server.domain.com On >
Now enter the same username and password as your incoming mail server in the user name and password field, where it states optional.
Now tap on Done to save changes.