The info is in the error message. I went to the URL in the error message and entered the IP address shown and this page displayed: https://check.spamhaus.org/listed/?searchterm=103.62.155.52 This indicates your PC is either affected with Malware or sending spam. The text also states that this is set by your email host, so you might check with them. It could be that some unwanted component is being attached to outgoing mail and my first suggestion would be to look at the message in sent folder to see if anything is attached, such as an ad by antivirus program or other. This is not an error of Thunderbird. Thunderbird is just displaying the returned message.