Problems / best practices for sending mails to Hotmail

[ontheair81]

Hello,

since today I have problems sending e-Mails to accounts at hotmail.com and hotmail.de. It was working fine before. This is the reply:

host eur.olc.protection.outlook.com[104.47.22.161] said:
550 5.7.1 Unfortunately, messages from [_my IP_] weren't sent.
Please contact your Internet service provider since part of their network
is on our block list (S3150). You can also refer your provider to
http://mail.live.com/mail/troubleshooting.aspx#errors.
[DB8EUR06FT025.eop-eur06.prod.protection.outlook.com] (in reply to MAIL
FROM command)

My IP is not on any blocklist, the server is not an open relay. SPF is correctly set, same as DKIM and DMARC. All of these tests passes when sending mails to any other mail provider (including gmail). Everything is perfectly fine when checking with mxtoolbox, for example.

I know, this is not a problem of DMS. And this is not a problem with DNS/SPF, etc.
After doing some research, it seems to be a common problem, that Hotmail/Microsoft is blocking smaller mailservers out of the blue - without any technical reason.
I already filled out a request at Microsofts EDFS formular, but according to other people, Microsoft is usually going to ignore these requests.

I would like to know, in which way you are dealing with this behaviour. Maybe there are any ideas on how to solve that, without relying on the arbitrariness of Microsoft?
As I said: I was able to send e-mails to hotmail before. The problem just started today, without any technical reason.

Any ideas or help is highly appreciated!

[ontheair81]

Additional information: Microsoft provides another website, where sending IP adresses can be delisted from the blocklist. Seems to be aimed towards Office-365-recipients and should work "within 30 minutes".

Maybe this could help some users.
But in my case: Nothing changed, my mails are still blocked by Microsofts weird "S3150 blocklist".

[casperklein]

is blocking smaller mailservers out of the blue

I don't think so. As the error message states, the whole network your IP address belongs to is currently blocked. Probably, because it was used in the past to send SPAM. If SPF etc. is correct, you don't have much options, beside trying your luck via the support form you mentioned.

[ontheair81]

the whole network your IP address belongs to is currently blocked

Oh, then I misinterpreted the initial error message! So Microsoft complains not about my specific IP, but about the whole IP range / provider network.

Just in case this could be helpful for other people, here is the update:
Some hours after filling the EDFS form, I received an automated message that my IP is "not qualified for mitigation" (will not be unblocked).
I replied to this message, providing informations about the configuration, which is following best practices to fight spam. Then a human at Microsoft checked these informations manually and replied. My IP will be unblocked.

So it looks like everything is fine now. But still not happy that Microsoft seems to block whole IP ranges, where I am the one who suffers, while doing nothing wrong ;-)