Please contact your Internet service provider since part of their network is on our block list

[#1]

This seems to happen after a period of weeks currently.  Webmail can send fine and after ringing up the help desk, that is all they are responsible for, so it's not an issue.

What I do not understand is why my mail service is reliable for months on end and then starts playing up.  This is the 3rd time it has happened.  Always when sending to Microsoft owned mail services.

Any ideas?

[#2]

See this topic, started three weeks ago, does it cover what you are asking about?

https://community.plus.net/t5/Email/Hotmail-rejecting-mail-from-PN-Diagnostic-Code-smtp-550-5-7-1/td...