If bounced emails are in the “Undeliverable” category, that means that the receiving email server is temporarily unavailable, was overloaded, or couldn't be found. A server that can't be found could have crashed or been under maintenance, so this may just mean waiting to send the email to the address again.

How do I fix undeliverable email?

Try these fixes: Make sure the recipient address is valid. Reduce the number of recipients in the message. If you received this error when sending a message using Outlook or another email app, try using Outlook.com to send the message instead.

Where do undeliverable emails go?

By default, any undelivered message is returned to the sender with an NDR and deleted from the queue.