553 you are trying to use me as a relay

I need a solution

Hi,

We have all inbound mail being routed through MessageLabs, then to our Office365 tenant.

All email is being delivered OK except for a specific circumstance.

Our domain is contoso.com.au.

If I create a DL, TestDL@contoso.com.au that has an external contact in it, user@yahoo.com.

If I send an email to that DL from my internal contoso address, it gets delivred OK.

If I send an email from an external address, eg from me@gmail.com to TestDL@contoso.com;

The email goes through MessageLabs
Then goes to Office365. Office365 sees that it needs to go back out to deliver to that external address.
Gets to MessageLabs, and MessageLabs returns "550 5.0.350 Remote server returned an error -> 553 you are trying to use me [server-17.tower-423.messagelab;s.com] as a relay, but I have not been configured to let you [104.47.116.56, mail-me1aus01lp2056.outbound.protection.outlook.com] do this. Please visit www.symanteccloud.com/troubleshooting for more details;about this error message and instructions to resolve;this issue. (#5.7.1)".

Within MessageLabs, we have configured the Office 365 hosted mail service in our outbound routes, so not sure what the problem is.

I've got cases open with both Microsoft and Symantec, but not getting very far.

Does anyone have any experience with this?

Thanks.