Thunar has a problem since many versions. Always after I open a file with a different viewer, this becomes the next default. Now I used "Send to -> Mail recipient" for a PDF file, next time normal clicking on it, it tries to send as email again. I don't think this is the expected behaviour.
Does ~/.config/mimeapps.list change every time you open the file with a different application?
Closing as duplicate. *** This bug has been marked as a duplicate of bug 14811 ***