I have 3 mail accounts defined in Thunderbird. say: [email protected], [email protected], [email protected]. 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 [email protected] or [email protected] 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 [email protected], it uses an incorrect certificate. In fact it uses the certificate for [email protected]. Also please note [email protected] 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.