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I came across this article which says that Microsoft is blocking SMTP from dynamically allocated public IP addresses.

https://support.microsoft.com/en-us/help/886692/microsoft-com-now-blocks-incoming-smtp-mail-from-servers-that-use-dyna

I don't know any mechanism that will tell whether an IP address on the internet is dynamically assigned or not. Reverse IP lookups may be able to give a clue with the name assigned to it but it would not be very accurate. I'm just curious , let me know what this can be.

yagmoth555
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user2629636
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  • Likely answer: They're likely blocking major residential ISPs on a case-by-case basis or based on behavior from the IP blocks. – ceejayoz Jul 06 '18 at 18:54
  • @ceejayoz They simply use the trick I told in my DVed answer. Residential dhcp range are tagged already. – yagmoth555 Jul 07 '18 at 03:29

2 Answers2

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They may, for example, use PBL from Spamhaus. See the first answer on Spamhaus PBL FAQ.

Tomek
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  • I'd agree with you, especially since the OP's linked article says "Note The bypass address that is mentioned in the 5.7.1 response is a special e-mail address that bypasses the dynamic IP check. The address will respond [...]. The auto-response also contains instructions on how to contact our real-time blocking list provider for updates to the list." However, your answer would be better if you could quote relevant parts of the FAQ or provide more context. – david Jul 06 '18 at 20:15
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Under IANA, regional registries such as ARIN (for the US) and RIPE (for Europe) will ask any IP block taker to know if it's for residential usage. This means that ARIN, RIPE and other registries willl know which IP blocks are dynamically allocated for residential use.

In big, an ISP can ask ARIn for an IP block, and ARIN can make sure those ip blocks remain in DHCP. It's to prevent running out of IP addresses, as space with static IP addresses are harder to control.

Note also that when an ISP needs more IP, ARIN can refuse to give them if they have too much static space used.

There are blackhole lists, such as the Spamhaus PBL. who will simply check that information, and if you're in a block allocated for dynamic addresses you'll be tagged. It doesn't make any difference if your ISP has reserved an IP address for you from their DHCP server or from PPPoE.

Edited:

You might also want to read Help! My IP address is on the PBL! What should I do?, which says:

Nothing, in most cases. Read through this FAQ for further explanations.

The first thing to know is: THE PBL IS NOT A BLACKLIST. You are not listed for spamming or for anything you have done. The PBL is simply a list of all of the world's dynamic IP space, i.e: IP ranges normally assigned by ISPs to broadband customers routers/modems (DSL, DHCP, PPP, cable, dialup). It is perfectly normal for these IP addresses to be listed on the PBL. In fact all dynamic IP addresses in the world should be on the PBL. Even static IPs which do not send mail should be listed in the PBL.

Jenny D
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yagmoth555
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