Plusnet outbound mail server (avasout-ptp-001) blocklisted by Microsoft

[#1]

I'm unable to send emails to Outlook/Hotmail/Live addresses. Details from the bounce message:

- Outbound mail server: avasout-ptp-001 [84.93.229.121]
- Blocked sending IP: 84.93.230.227
- Error code: 550 5.7.1
- Reason given by Microsoft: "part of their network is on our block list (S3150)"
- Microsoft's reference link: http://mail.live.com/mail/troubleshooting.aspx#errors
- Date/time of failure: 28 June 2026, 13:26 UTC
- Recipient affected: an outlook.com address
- Microsoft tracking ID: 08DED23C57966AD6

I understand that avasout-ptp-001 has been blocklisted before (e.g. by Spamhaus), and that this has previously required an internal incident report to get it delisted. It looks like the same server may be on a Microsoft blocklist this time.

Any suggestions?

[#2]

Is your SMTP server set to relay.plus.net? If you have been migrated to Greenby, you could try changing it to relay.force9.net which avoids going via Plusnet's avasout servers. If you haven't been migrated to Greenby then I think the technical term is 'you're stuffed'.

[#3]

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@Idleman wrote:

I'm unable to send emails to Outlook/Hotmail/Live addresses. Details from the bounce message:

- Outbound mail server: avasout-ptp-001 [84.93.229.121]
- Blocked sending IP: 84.93.230.227
- Error code: 550 5.7.1
- Reason given by Microsoft: "part of their network is on our block list (S3150)"

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'Interesting' reading: What does error "S3150" mean? - Microsoft Q&A

I understand that avasout-ptp-001 has been blocklisted before (e.g. by Spamhaus), and that this has previously required an internal incident report to get it delisted. It looks like the same server may be on a Microsoft blocklist this time.

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Clean at the moment, according to Spamhaus: https://check.spamhaus.org/results?query=avasout-ptp-001-m.plus.net
avasout-ptp-001-m.plus.net "has no issues"

However: 

https://dnschecker.org/ip-blacklist-checker.php?query=avasout-ptp-001-m.plus.net
"Can not fetch DNS records of provided domain."

https://dnschecker.org/dns-record-validation.php
"Can't find auth DNS server for avasout-ptp-001-m.plus.net"

Any suggestions?

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Yeah. Nuke Microsoft?

[#4]

The elephant in the room is Plusnet's SMTP server is an Open Relay.

[#5]

Thanks all.  Changed my ancient smtp.care4free.net to relay.plus.net and can now reach Microsoft addresses.  Am still being rejected by tiscali, but judging from other posts this looks like 'a thing'