What is 'Blocklist 1, mail from IP banned'

Soldato
Joined
11 Feb 2004
Posts
4,532
Location
Surrey, UK
All emails we send to a specific company are returning this message:

xxx x.x.x Service unavailable; Client host [xxx.xxx.xx.xxx] blocked using Blocklist 1, mail from IP banned; To request removal from this list please forward this message to [email protected].

Up until this week we could send OK. Same thing happens when using Internet Email or web mail (e.g. Hotmail, Just Host etc)

Where do I start?
 
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