Sometimes it can happen that digitally signed emails sent by Thunderbird show an invalid signature at the recipients. One reason may be the way Thunderbird formats emails. By changing a setting ("mail.strictly_mime" = "true") of the message, the sender can fix the problem.
 

1. Open "Thunderbird settings".

   

2. Search for "Edit configuration...".

   

3. Search for mail.strictly_mime and click the arrow icon at the end of the entry.
   Attention, make the change only to the entry which is exactly named mail.strictly_mime.

   

4. The mail.strictly_mime entry should be set to "true".