I have exactly the same problem with Windows 7 x64 and Outlook 2007 in Outlook Only Mode.
If I deaktivate the "Comodo SecureEmail Core" Plugin at Outlook the problem is gone. But incoming emails are no longer decyiptet automatically.
I am using Outlook 2010 on Windows Seven x64 (32bit office software) with the Comodo Plugin only option. Outlook kept forgetting my settings for the signing and encrption certificate in
>file
>Options
>trust center
>trust center settings
>e-mail security
>settings
I disabled the "Comodo Secure Email Core" and mine is now working fine.
I have comodo version: 2.5.0.31
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