Cannot Send Mail The server rejected one of the recipient's addresses. The message has been placed in your Outbox.

Hello toadgolf,

Thanks for using the Apple Support Communities! We understand you are getting an error message when trying to send an email out. We are happy to help you find some information.

Here is a link to assist you.

If you can’t send email on your iPhone, iPad, or iPod touch

If you are still having an issue after reviewing the above link, please let us know. Also, can you tell us how long you have had this issue? Have you verified that the email you are having an issue with is valid?

Have a great day!