Thunderbird is not causing this - it is simply reporting that the IP listed has been blocked from sending email. That message about spamhaus means that either your local network or device, or your email server has been blocked because of sending spam or suspicious emails. If 208.181.95.34 is your own local IP address, then you need to figure out how your system is sending out spam, stop the issue, and apply to Spamhaus to have your IP unblocked. If that IP address belongs to your email service provider, then they need to know that they are blocked and must clear their reputation to get unblocked.

Thanks. I contacted Spamhaus and the blocking was removed. It seems to have been an issue with my computer not my ISP but I have no idea how to stop the sending of spam or suspicious emails. I will do a deep virus scan to see what might turn up.

A virus scan found nothing. Is there a Thunderbird tool I can try to find out what caused the alleged spam or suspicious emails? I am concerned this blockage will occur again. Despite that blockage of my computer I was able to send email using the ISP's mail function.

It's nothing to do with Thunderbird, so no, there are no tools. It simply delivered the response from Cloudfilter. Also, if you have several devices on the same network it may not even be coming from the computer you have Thunderbird on. Spamhaus has some helpful articles about preventing outgoing spam, or you may need local tech to inspect your network traffic and devices. Your ISP might be able but probably won't check your outgoing traffic for clues. Sorry we can't help more but investigating an invading spam bot is beyond this forum.