Please check to see if you have an exception listed.

Thunderbird >Preferences > Privacy Mail Content section - click on 'Exceptions' button. select any you want removed click on 'Remove site' click on 'Save Changes' click on 'OK'

re :Thunderbird tries to connect to click.mail.[domainname].com. Please elaborate on what you mean by 'tries to connect to'. Did you see that info in the bottom Status Bar ? Did you move the mouse pointer so it hovered over a link? Did you click on a link?

Thank you, the domain was indeed listed under exceptions.

Re your second post, I have Little Snitch installed which displays a popup when programs try to connect to a server. So when opening the mentioned email, I got the respective alert.

I thought that the yellow content blocking banner in an email means all content is blocked, but this seems to be wrong? When one of several domains is an exception, that content is accessed while the banner still displays, correct?

re:When one of several domains is an exception, that content is accessed while the banner still displays, correct?

Yes, because that particular remote content has been allowed by you at some point in the past. Emails sent with remote content can vary in the actual remote content they contain if the sender changes it. That particular remote content may have been previously permitted from either the same sender or another sender. In the opened email, where it says ("to protect your privacy, Thunderbird has blocked remote content in this message."), click on 'options' and it will list all the various remote content contained in the email.

Thanks a lot for confirming. I think the banner is misleading to users, but now I understand the concept.