To send emails, I use one SMTP server, say domain1.com. But I use two identities that set up in Thunderbird, say firstname.lastname@example.org and email@example.com. For those who are not knowledgeable about it, this means that my addressees see emails coming from firstname.lastname@example.org and from email@example.com, with no evident link between the two. Furthermore, I was issued a SMIME certificate for each identity from an authority, in the form of a .p12 file. This permits me to digitally sign my emails.
How do I set up Thunderbird so that it signs my outgoing emails with the correct certificate respectively to each identity? The problem I now face is that I know only how to associate my SMTP server to a certificate, but not to each identity separately.