Outlook rejecting emails from relay.plus.net

[#1]

I can't send to outlook email addresses today. Give this:

Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.17] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

[#2]

Are you attempting to send from on the Plusnet network or via some other connection?

Which email brand?

Please confirm that you have authentication set on the SMTP server.

If the SMTP server is set up with authentication, then this is another instance of Microsoft playing hard ball with Plusnet.  I have seen similar where the AWS platform is the sending mail service.

[#3]

port 587 / SmartTTLS / On plusnet network. I tried to my own outlook address and it did same.

I just tried AGAIN and it's not bounced .. so far.. I'll try to my own address and .. ok it's arrived!

Must've been a "blip" at outlook? Strange though as to my original person I wanted to mail and to myself it gave that exact same error immediately, 5 minutes apart. Full error was :

Final-recipient: rfc822; !redacted!@outlook.com
Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [212.159.14.17] 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. [Name=Protocol Filter Agent][AGT=PFA][MxI= blah blah

[#4]

The from address can have a massive bearing on these issues.  Which brand are you sending from - one of the Plusnet brands or your own domain name?

Which relay server are you using?

All the details make significant differences to resolving the problem.

[#5]

Oh it's resolved now, as I say it started working again.

Just a blip presumably at outlook. I'd not changed anything, has worked for years, stopped earlier and is working again.