Your message to user@domain.com couldn’t be delivered.

A custom mail flow rule created by an admin at domain.com has blocked your message.

<more stuff here>

More Info for Email Admins
Status code: 550 5.7.1_ETR

This error occurs because an email admin at domain.com has created a custom mail flow rule that has blocked the sender’s message.

It would be great if these NDRs would tell you plainly which Exchange Online rule blocked an email. Here’s how you can find out.

1. Run a message trace in the Exchange Online. It will give you a Mail flow rule ID number like this: “Mail flow rule: ‎F6B2848C-379D-41C9-83FB-AB0AC14F9A5A‎”. You can also find the same information in the “Message events” section like this: “Transport rule: ”, ID: (‘F6B2848C-379D-41C9-83FB-AB0AC14F9A5A‎’), DLP policy: ”, ID: (00000000-0000-0000-0000-000000000000).”
2. Connect to Exchange Online via PowerShell and run this cmdlet, where <transport rule id> is the ID number returned by the message trace:
   Get-TransportRule | ?{$_.guid -eq “<transport rule id>”}

This will give you the name of the mail flow rule, which you can then go and disable or adjust as required.