O365 Mail Routing (Hop count exceeded - possible mail loop ATTR1)

I'm experiencing some issues in out Office 365 Hybrid environment...

Strangely, out of the few thousand users, there is a very small handful of users that people who have mailboxes in O365 get the following NDR when emailing them.


Sender:                Cal.Naughton@contoso.com (mailbox in O365)
Recipient:            Ricky.Bobby@contoso.com (Mailbox On Premises - myexch2013server.contoso.com)


BN3NAM01FT032.mail.protection.outlook.com rejected your message to the following e-mail addresses:
Ricky.Bobby@contoso.mail.onmicrosoft.com

BN3NAM01FT032.mail.protection.outlook.com gave this error:
Hop count exceeded - possible mail loop ATTR1 [BN3NAM01FT032.eop-nam01.prod.protection.outlook.com]
A problem occurred during the delivery of this message. Please try to resend the message later.

Diagnostic information for administrators:
Generating server: myexch2013server.contoso.com
Ricky.Bobby@contoso.mail.onmicrosoft.com

BN3NAM01FT032.mail.protection.outlook.com #554 5.4.14 Hop count exceeded - possible mail loop ATTR1 [BN3NAM01FT032.eop-nam01.prod.protection.outlook.com] ##rfc822;Ricky.Bobby@contoso.mail.onmicrosoft.com


Recipient SMTP:Ricky.Bobby@contoso.com does have an email alias on-prem of smtp:Ricky.Bobby@contoso.mail.onmicrosoft.com but doesnt have a Target Address stamped. So I would think it wouldn't be sending the message back to ExOL etc.


I don't know where to start troubleshooting, this is only happening when cloud users try emailing to 5 users that are on prem. SO that would rule out DNS and connectors I would think, since it works fine for the other thousands when sending On-Prem > Cloud and Cloud > On-Prem.

Any ideas? Thanks in advance.