Re: Can't send approval emails - The recipient's email provider rejected it.
Hi @Anonymous,
Could you please share a full screenshot of your flow's configuration?
Do you send an approval email to the external user using the "Send email with options" action?
The error message told that the email message was rejected by one of the servers trying to deliver it. I have made a test on my side and don't have the issue that you mentioned. my flow's configuration as below:
The flow works successfully as below:

Please check and see if the following article would help in your scenario:
https://kb.intermedia.net/article/3243
Best regards,
Kris