CSE leaves message in Outbox with Outlook 2007

I’m running CSE as a plugin to Outlook 2007 (32-bit) under Vista (32-bit). I find that (almost) every message ends up in the Outbox and claims to be unsent, even though it’s also in the Sent Items folder and the recipient has received it (and replied in many cases). I run multiple email accounts in Outlook, but only one has a certificate (I tried to keep multiple certificates and it caused endless problems, so only one account signs). Those with no certificate, after I respond to the dialog box asking if I want to send unsigned and unencrypted, never generate an Outbox copy of the message - only the account with the certificate, where the outbound message is signed.