Email to Hotmail being rejected

[#1]

I use Mac mail with an icloud address and sending to a hotmail address, one of the email has bounced back - I tested it on my son's hotmail email and that didn't bounce back.

Any advice? I contacted PlusNet who advised changing my server settings, but not sure how to do that on Mac mail?

This was the error message - 

host eur.olc.protection.outlook.com[52.101.73.17]
   said: 550 5.7.1 Unfortunately, messages from [57.103.86.31] weren't sent.
   Please contact your Internet service provider since part of their network
   is on our block list (S3150).

[#2]

I use Mac mail with an icloud address and sending to a hotmail address,

You are sending from an icloud address via an Apple server, the problem is nothing to do with Plusnet. You need to contact Apple since it is their server , 57.103.86.31 , which is on the blocklist.

Any advice? I contacted PlusNet who advised changing my server settings, but not sure how to do that on Mac mail?

Changing your server settings wont help...

[#3]

Ah ok I see - thanks - I'll get in touch with Apple

[#4]

Ok something odd is happening.  I've re-tested the email and sent it through ok this time.  Has anyone had this as a sporadic problem - no-one has fixed anything at either end as far as I'm aware....?