Hi Mahesh,
To find the root cause, could you send me the entire error message to me via a private message?
I've sent a private message to you to request the error message.
Here are the steps to see the private message:
1. Please go to your details section on the right side of the community site.
2. Click Private messages.
3. Click the subject title of the response to read the message.
In addition, I'd like to collect the following information:
1. Whether the sender address of the error message comes from Microsoft.
2. Whether all users in the distribution group received the email when the customer received the error message.
3. Please double check whether there are any rules has been created on the Exchange admin center of Office 365.
Sign in with the admin account -> "Admin" -> "Exchange" -> Navigate to "Mail flow" -> "rules"
I'll appreciate it if you could post the information here.
Thanks,
Aaron Yuan