Funkiness with "This message signed by..."

I’ve just installed an s/mime certificate, and when i send an email to a test account (which is received by the same installation of eM Client) the signed email says:

“This message is signed by 's COMODO Limited CA ID.”

However, after doing other things and coming back to the email, this has changed to:

“This message is signed by [my email address].”

The first is clearly wrong, but the second is correct. What’s going on here?