Can't send mail - The recipient was rejected by the server

Check the outgoing mail server setting. Make sure that your username and password are in there.

Settings>Accounts & Passwords>Your email account>Account>Outgoing mail server - tap the server name next to SMTP and check in the primary server and make sure your username and password are entered and correct - even if it says that the password is optional.

That message means that you do not have a username and password for the OUTGOING mail server settings on your phone. Your username and password must be entered twice if you are manually configuring your email account; once for incoming, and again for outgoing.

Also check for multiple outgoing mail server settings. One of them may not have the proper credentials, even if others do.

longtimemacuser666 wrote:

Most - but not all - of the email I attempt to send from my iPhone X fails, with the error:

"Cannot Send Mail. A copy has been placed in your Outbox. The recipient "<recipient's email address>" was rejected by the server because it does not allow relaying."

When I get this message, sometimes (not always) the email is actually delivered to the recipient (I know because they replay back to me). Sometimes it doesn't get delivered at all.

Sometimes I send mail to one of these people and the mail goes right through with no error.

*** is happening, and why is this such a terrible user experience?

Most of all, how do I fix this so "it just works"?

iPhone X running iOS 11.2.5, and the phone's default mail app.

What did your mail provider say when you contacted them about the issue?

The next thing that I would try is to delete the account and start over again.