Unable to send e-mails to Outlook e-mail account users

[#1]

Although this problem has been mentioned previously in e-mail problem threads, I thought I'd start a new thread to see if anyone, superusers or whoever, can engage with technical staff at Plusnet/Greenby to implement a solution. This problem is causing me significant disruption.

e-Mails to Outlook.com addresses from my PC using Outlook365 as part of Office365 suite are being bounced with the following:

Reporting-MTA: dns; avasout-ptp-002 [84.93.230.235]

Diagnostic-Code: smtp; 550 5.7.1 Unfortunately, messages from [84.93.230.235] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150).

Surely Plusnet/Greenby tech staff can engage with Microsoft tech staff to implement an unblocking?

[#2]

Using Webmail I got an identical error reported when trying to reply to a hotmail.co.uk address earlier today Sunday 7th Dec at 11:01hrs

The system was happy enough sending to the same account on Friday...

[#3]

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@ebforum wrote:

Surely Plusnet/Greenby tech staff can engage with Microsoft tech staff to implement an unblocking?

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According to @MisterW here:

https://community.plus.net/t5/Email/Diagnostic-Code-smtp-550-5-7-1-part-of-their-network-is-on-our/m...

they are.