By the way I should add that I have tried opening T-bird in Safe mode, and with add-ons disabled, neither of which make any difference. I have also reconfirmed my browser in default programmes.

Mailchimp is adding extra unnecessary code into the link and confusing Thunderbird. In more detail, it is adding target="_blank" to the mailto: link.

The problem can be avoided at the MailChimp end by unticking the "Open in a new window" option when adding mailto: links to a campaign email. By default that option is ticked when in fact it shouldn't be there at all. Of course this is no solution for the recipient of a badly formatted MailChimp email.

If you have the option to view the email in a browser, then the mailto: links work there.

MailChimp should update their software so that it doesn't add unnecessary code.

Thunderbird could update their software to ignore the unnecessary code.

Mailchimp is adding extra unnecessary code into the link and confusing Thunderbird. In more detail, it is adding target="_blank" to the mailto: link. The problem can be avoided at the MailChimp end by unticking the "Open in a new window" option when adding mailto: links to a campaign email. By default that option is ticked when in fact it shouldn't be there at all. Of course this is no solution for the recipient of a badly formatted MailChimp email. If you have the option to view the email in a browser, then the mailto: links work there. MailChimp should update their software so that it doesn't add unnecessary code. Thunderbird could update their software to ignore the unnecessary code.

I have the same issue with any link. Not only mailchimp, simple mailto-Links on different websites. Sometimes there are hidden with javascript, most of them aren't. Any idea?

Thank you -- you are quite right. When I also interrogated the code (as a raw amateur) I found the errant addition and removed it. I still want emails opened in a new window - otherwise the reader moves away from the 'Chimp' original... Thank you for putting it so helpfully!