How about posting the exact messes you get, verbatim. My guess on almost no information is that it is unlikely to have anything to do with Thunderbird and everything to do with how you configured it long ago and changes with the server that make those old configurations invalid. But we know nothing of the provider that is rejecting the mails, and recent changes with that provider, like the ones that have occurred with Spectrum. SO I am reduced to googling the error "[C20]: message blocked" and the results returned indicate the error may originate with an ISP support company mailchannels. https://support.mailchannels.com/hc/en-us/articles/202223160-Fixing-the-550-5-7-1-CS-Message-blocked-error (only on their web site did google find an entry with the [C20 part of the error message) SO this may well be a misconfiguration by your internet service provider of their mail servers or a long standing misconfiguration of Thunderbird that they are now detecting and refusing to accept. If you are sending from a single mail account, my initial guess is you need to read the mailchannels document and perhaps talk to your ISP.