Three times today, I've seen bounced messages with "mail forwarding loop for <destination address on our servers>" that shouldn't be happening (the destination addresses were fine and had no apparent connection).
Some investigation showed that those messages already had a "Delivered-To: <destination address on our servers>" header before they ever reached our servers. This breaks Postfix mail loop detection, which uses that header to tell if a message has already been delivered [1]. The messages came from Amazon corporate mail servers -- two were from addresses @amazonstudios.com, and another @corpmail.amazon.com. The messages each go from Microsoft, to Amazon's servers, and then to our servers. Assuming the headers aren't being re-ordered, the Amazon servers are adding the "Delivered-To". Here's an example of the relevant headers of one before it reached our servers (the "Delivered-To:" address is redacted, although it exactly matched the "RCPT TO:" recipient address): --------------------------------------------------------------------- Received: from ip-10-4-17-41.ec2.internal (HELO smtpout.naws.us-east-1.prod.farcaster.email.amazon.dev) ([10.4.17.41]) by internal-iad-out-005.esa.us-east-1.outbound.mail-perimeter.amazon.com <http://east-1.outbound.mail-perimeter.amazon.com/> with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 04 Feb 2026 18:43:10 +0000 Delivered-To: [redacted-recipient-address] Received: from EX19MTAUEA001.ant.amazon.com [52.94.133.131:10896] by smtpin.naws.us-east-1.prod.farcaster.email.amazon.dev [10.0.36.81:2525] with esmtp (Farcaster) id 98ec55e9-886f-4850-9178-11df7239c275; Wed, 4 Feb 2026 18:43:10 +0000 (UTC) X-Farcaster-Flow-ID: 98ec55e9-886f-4850-9178-11df7239c275 Received: from EX19EXOUEC001.ant.amazon.com (10.252.135.173) by EX19MTAUEA001.ant.amazon.com (10.252.134.203) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.35; Wed, 4 Feb 2026 18:43:10 +0000 Received: from EX19EXOUEC002.ant.amazon.com (10.252.135.179) by EX19EXOUEC001.ant.amazon.com (10.252.135.173) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.35; Wed, 4 Feb 2026 18:43:09 +0000 Received: from BN1PR07CU003.outbound.protection.outlook.com (10.252.135.199) by EX19EXOUEC002.ant.amazon.com (10.252.135.179) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA) id 15.2.2562.35 via Frontend Transport; Wed, 4 Feb 2026 18:43:09 +0000 Received: from MN0PR18MB5919.namprd18.prod.outlook.com (2603:10b6:208:4a6::13) by DS0PR18MB927468.namprd18.prod.outlook.com (2603:10b6:8:310::11) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9587.13; Wed, 4 Feb 2026 18:43:07 +0000 Received: from MN0PR18MB5919.namprd18.prod.outlook.com ([fe80::6b96:39f5:56ff:6d8e]) by MN0PR18MB5919.namprd18.prod.outlook.com ([fe80::6b96:39f5:56ff:6d8e%4]) with mapi id 15.20.9587.013; Wed, 4 Feb 2026 18:43:07 +0000 From: <[redacted-sender]@amazonstudios.com> --------------------------------------------------------------------- If you're Amazon, you might want to fix this. If you're expecting email from Amazon, you might want to be on the lookout for this. [1] <https://www.postfix.org/local.8.html>: "If mail arrives for a recipient that is already listed in a Delivered-To: header, the message is bounced." -- Robert L Mathews
_______________________________________________ mailop mailing list [email protected] https://list.mailop.org/listinfo/mailop
