What version of Comodo SecureEmail have you installed?
This error is shown by Outlook if this mail client can’t find certificate for signing/encryption.
If you use released version (currently: 184.108.40.206) this version has just LSP mode.
That means that CSE is encrypting/signing e-mails manually and you have to turn OFF encryption and signing in your mail client (Outlook in this case) to let it send unencrypted and unsigned e-mails and then CSE will intercept these e-mails and will sign/encrypt them according to CSE options.
I hope this post will help to resolve your issue.
Feel free to ask me any questions on CSE.