> P.S. Alessandro, could you, please, response to mails using "reply"
> button. You messages lack of In-Reply-To and References headers, so at
> least thunderbird does not group messages into a thread.

Sure, no problem. I usually use vanilla compose-mail to answer,
probably that's the problem. I am now trying directly with Gmail, let
me know if it's working now.

As I wrote in a previous answer, right now I simply set up Citar to
use Sioyek by default:

(setq citar-file-open-function '(lambda (file)
 (call-process "sioyek" nil 0 nil
 file)))

This is not a problem since, when opening a citation, I would most
likely want to use Sioyek for it. However, you gave me nice ideas to
further explore. I'll have a look at mailcap and see if the script is
worth being set up. I'll let you know if I discover something useful!

In any case, thanks for your answer!

Alessandro