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I'm struggling with an issue regarding hosts connecting to my SMTP server.

For instance, one client is connecting to my SMTP server with the given hostname : EUR02-AM5-obe.outbound.protection.outlook.com.

Unfortunately, the IP doesn't match that hostname (52.101.131.25).

Based on the SMTP rules, I should refuse this email, but when I do a reverse IP lookup for 52.101.131.25, I get mail-bgr052101131025.outbound.protection.outlook.com, which also comes from Outlook.com !

So, even if the hostname doesn't match the IP, the IP is originating from a valid source in that case (outbound.protection.outlook.com).

Unfortunately I can't go up between the two hostnames to find a common operator, because at some point, I could validate ".com" agains't ".com".

And Outlook is not the only one doing so, here's the case with Zoho:

HELO sender21-mail.zoho.eu coming from the IP 185.20.209.254. But when you do a reverse lookup, you don't find sender21-mail.zoho.eu, but ... sender.zoho.eu

How can I do to ensure that both hostname come from the same valid source, that's just the IP that is badly configured?

Thanks!

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