I actually do not see a connection between Thunderbird and the process used by some mail servers to use blacklists to filter mail. Blacklisting is a somewhat hit and miss filtering used by many email recipient organizations to refuse mail from certain IP addresses, providers and reported spamming addresses (and sites hosted in the same location) Periodically we see reports here of internet service providers (ISPs) making such a mess of their use of blacklists that they manage refuse to accept emails from their own retail customers when they submit email . Interestingly the reports here appear to only originate in the USA where most of the providers are trying to sell advertising on webmail sites. But that might be just coincidental. But I have never seen a report of such outside the USA. Clearing the issue might be as simply as changing your mail provider because they don't care and are blocking your outgoing mail. Or it might involve getting the IP address of your mail server changes or in the case of web site hosting having your mail moved to another server. It might also be that you need to stop sending SPAM. However without specific details of your case, this topic could fill an encyclopedia without covering anything actually relevant to you specifically.