Thunderbird is not generating that message. If you go to the link it tells you this: ''To provide a high quality service to our customers, we monitor the quantity and type of emails being sent to our servers. If a mail server is found to be sending large quantities of email that we have categorised as spam to our servers, we will block its IP address.'' ''Fasthosts uses Realtime Blacklists (RBLs) to check that email is sent from a legitimate source. If an IP address has been blacklisted, the decision on whether to accept the email for delivery is not based upon the content on the email, the sender's email address, or the recipient of the email. It is based solely on the mail server that is being used by the sender.'' At the link they mention for your convenience: https://antispam.fasthosts.co.uk/ There is also a box where you can test. It says: Enter the IP address of your mail server in this box to see if it is currently blacklisted. So I entered the IP address you mention and it is blocked https://antispam.fasthosts.co.uk/86.154.112.44/ So I noticed it was set by 'Spamhaus' - more details link goes to this: https://check.spamhaus.org/listed/?searchterm=86.154.112.44 There is also a 'Sorbs' - more details link - clickon it to get info You need to follow instructions at those locations to get it removed.