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Spam checking issue with IPv6
Problem reported by Kayasidh Kasyapanun - 3/18/2026 at 8:24 PM
Resolved
K
We've seen problems with senders whose originating their mails with IPv6 such as outlook.com
SPF and Reverse DNS fail to check for those mails. Here is an example.

THIS IS THE MAIL HEADER
Received: from TYPPR03CU001.outbound.protection.outlook.com (mail-japaneastazon11022097.outbound.protection.outlook.com [52.101.126.97]) by ourmailgateway with SMTP;
   Thu, 19 Mar 2026 09:45:33 +0700
Received: from JH0PR01MB6874.apcprd01.prod.exchangelabs.com
 (2603:1096:990:7d::10) by TYZPR01MB6076.apcprd01.prod.exchangelabs.com
 (2603:1096:405:78::11) with Microsoft SMTP Server (version=TLS1_2,
 cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.9723.19; Thu, 19 Mar
 2026 02:45:07 +0000
Received: from JH0PR01MB6874.apcprd01.prod.exchangelabs.com
 ([fe80::6ed5:f3a1:6d2a:c04e]) by JH0PR01MB6874.apcprd01.prod.exchangelabs.com
 ([fe80::6ed5:f3a1:6d2a:c04e%5]) with mapi id 15.20.9723.018; Thu, 19 Mar 2026
 02:45:07 +0000
THIS IS SPAM CHECK LOG
[2026.03.19] 09:45:41.087 [17277862] Spam check args: from: sender@thedomain; messageID: 17277862; messagePath: C:/SmarterMail/Spool/SubSpool5/17277862.eml; sender: sender@thedomain; sendersDomain: thedomain; sendersIp: fe80::6ed5:f3a1:6d2a:c04e; returnPath: sender@thedomain; sendersEhlo: TYPPR03CU001.outbound.protection.outlook.com
[2026.03.19] 09:45:41.154 [17277862] [fe80::6ed5:f3a1:6d2a:c04e] No valid reverse DNS entry found.
[2026.03.19] 09:45:41.250 [17277862] Running SPF check
[2026.03.19] 09:45:41.328 [17277862] SPF Fail. IP: fe80::6ed5:f3a1:6d2a:c04e, Sender: sender@thedomain, FailReason: [no matches for fe80::6ed5:f3a1:6d2a:c04e]; all result of Fail observed

I have to lower SPF and Reverse DNS scores to allow these good mails to arrive.

How can I fix this?

Thank you in advance.
Sébastien Riccio Replied
3/19/2026 at 5:52 AM
Strange indeed. I think it should do the SPF check on 52.101.126.97 which seems to be the last hop before it reaches your server.

For some reason it seems it's doing the check on fe80::6ed5:f3a1:6d2a:c04e which are non-routable link-local IPv6 addresses, it makes no sense :(
Sébastien Riccio System & Network Admin https://swisscenter.com
K
Kayasidh Kasyapanun Replied
3/19/2026 at 6:24 AM
Note that the IP of outlook.com (52.101.126.97) is whitelisted on the server. I have these options checked in whitelist page.
- Bypass IP for Spam Checks
- Bypass Greylisting
all other options are unchecked.
Sébastien Riccio Replied
3/19/2026 at 7:19 AM
Marked As Resolution
Oh then, that's the bypass IP for spam checks in action. It skips the ip for SPF check and take the next one in the received lines.

It's mainly used to whitelist incoming gateway adresses so SmarterMail doesn't check SPF against your incoming gateway IP address.
Sébastien Riccio System & Network Admin https://swisscenter.com
K
Kayasidh Kasyapanun Replied
3/19/2026 at 11:02 PM
Thank you so much.
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