Email won’t send - server rejected recipient email address

I’ll assume this occurs across multiple different recipient addresses; that no messages are being sent.

If so... Your SMTP server settings are probably wrong. Mail has two groups of settings, one for receiving and reading, and a second and separate set for sending mail. Each group contains its own access credentials, server name, amd provider-specific settings. One group uses POP or IMAP for receiving and reading, and the ohter group uses SMTP or ESMTP for sending. Your sending-related details are likely incorrect. You’ll probably need to re-enter those, after verifying those match the requirements of your particular mail provider.

If you can’t send or receive emails on your iPhone, iPad, or iPod touch - Apple Support

Check with your email provider, around the required settings. Many of the providers can see into the mail server logs, and can usually see what the error is. Also have somebody else work through the settings, and check for errors. Verify the settings on a separate device or computer, as well.

Garethlepage wrote:

Thanks for the feedback - all worked well before the software update, so not sure if settings need to be changed?

And now it doesn't work, right?

Does this not work for one user, or do all messages get stuck when sending?

If one message gets stuck, check that the recipient email address is valid.

If all messages get stuck, then either some related detail or requirement has changed at the server—which can be entirely independent of your software update, such as dropping support for insecure connections, or changing the SMTP server host name and as I've experienced here—or the iPadOS software update or some other change has caused a corruption with the password, or with some other part of the SMTP-related server settings.

If all messages get stuck, then follow the troubleshooting for the send path.

If all messages get stuck, check your settings against those for your particular mail provider; specifically the SMTP-related send-related configuration.

This could be that you have limited out on this email thread. Meaning the thread may be limited to say 100 replies. This is done to save space and speed up the servers. Try deleting the email thread and starting another. If this is your first email in the thread then belay my last paragraph. Good luck.

The error message doesn’t say that you cannot send email - but instead suggests that something is wrong with [one of] the recipients email address to which you are trying to send mail.