E-Mail Blocked - 550 5.0.350 Code

[Bill Anderton]

We are a regional law firm in Dallas (tkach-law.com). All of our transaction mail (one-on-one e-mail -- NOT e-campaigns which we only do very rarely) being sent out are being blocked by your product for our 4,000 clients that use Yahoo! Mail addresses.

The following messages are returned on the e-mail that bounces back:

550 5.0.350 Remote server returned an error -> 553 you are trying to use me [server-21.tower-267.messagelab;s.com] as a relay, but I have not been configured to;let you [104.47.36.58, mail-sn1nam02lp2058.outbound.prot;ection.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)

We use Microsoft's Hosted Exchange product, fully set up, checked and working well for several years. SPF, DMARC and DKIM have been set up for years.

DSN generated by:	DM5PR16MB1657.namprd16.prod.outlook.com
Remote server:	server-21.tower-267.messagelabs.com

Headers show we passed authentication checks:

ARC-Authentication-Results: i=1; mx.microsoft.com 1; spf=pass smtp.mailfrom=tkach-law.com; dmarc=pass action=none header.from=tkach-law.com; dkim=pass header.d=tkach-law.com; 

We do not appear on any global blacklists that I can find.

[Bill Anderton]

I have been through our Hosted Exchange configuration completely and found no issues/problems on our side. Microsoft engineers have reviewed our set up on the phone for an hour this morning and ran complete extended diagnostics and found nothing wrong on our side. Collectively, we simply can't see the cause for the relay issue on our end. I have tested with mail from other Hosted Exchange domains we own or operate with the exact same configuration as this domain and delivery is okay; the problem is with only mail originating from this domain (tkach-law.com.)

Without anything else to go on, it would appear this relay issue is inside messagelabs.