Environment: Thunderbird Portable 9.0.1 on Windows XP Pro (32 bit).
Thunderbird Portable is run from the local hard drive (not from a USB one).
Thunderbird Portable has been upgraded from earlier versions gradually.
Installed automatically (from TERENA Personal CA) a personal certificate in Firefox Portable 9.0.1. I created certificate backup and imported into Thunderbird Portable (9.0.1), then I enabled in an account signing and encryption using this certificate. The account includes copied mail. Signed messages sent using this account cannot be verified.
THIS PROBLEM DOES NOT OCCUR WITH A FRESH INSTALLATION OF THUNDERBIRD (NOT PORTABLE). It also does not occur with other mail clients (e.g. MS Outlook).
However, using other certificates, like the ones generated here: https://digitalid.verisign.com/client/class1Netscape.htm, we can create verifiable messages. (Edit: This, after subsequent investigation, seems irrelevant. The problem is with the account, not with the certificate.)
Why Thunderbird Portable cannot send a signed mail message which can be verified successfully, when mail folders have been copied (migrated) from elsewhere (another TB installation)? And why this does not happen in Thunderbird (standalone)?