We had a customer getting this bounce back error occasionally. Research lead me to an Exchange distribution group restriction.

I went into the exchanged and examined the Group Distribution List. I wasn’t able to just add the email address so I left it checked with “Accept messages from”  and checked “All senders”. I then unchecked “Require that all senders are authenticated.” I used the following article to help me with this call.

https://kb.intermedia.net/article/3278

My final screen looked like this.

Let us know if this helps you out….