I have 3 mail accounts defined in Thunderbird. say: user1@mail1.com, user2@mail2.com, user3@mail3.com. For each I have registered in the Thunderbird settings, 1 certificate - So 1 for mail1.com account, 1 for mail2.com account and 1 for mail3.com account. All valid. See attachement cert 1 When i send an email from user2@mail2.com or user3@mail3.com it all works well... I check the email on the receiving end and it is signed with the proper certificate - all great. However, when sending an email with user1@mail1.com, it uses an incorrect certificate. In fact it uses the certificate for user3@mail3.com. Also please note user1@mail1.com is the first email in the list of accounts. I have tried and removed and added the certificate again thinking it's a glitch. But it works for a while and then same problem happens again. Any ideas? I have a feeling it's a bug since thunderbird is picking the wrong certificate in the list.