Hi @JamesHC. Thanks for posting. It's hard to say for sure why this is happening without more information. Typically, if a message is rejected by a recipient's email provider, are rejection notice (or bounce back) is sent to the sender. If you have that, feel free to share it. It will likely indicate why the issue is occurring. If you're not seeing a rejection notice, then the recipient should probably reach out to their provider to see if they can find out why the messages aren't coming in.
451 Sorry, I wasn’t able to establish an SMTP connection. I’m not going to try again; this message has been in the queue too long.
Cause of error:
I hope that helps.