Hi,
Please check out our email certificate FAQs at:
https://support.comodo.com/index.php?_m=knowledgebase&_a=view&parentcategoryid=32&pcid=0&nav=0This sounds like you have not stored, in the address book, the certificate for the person you are trying to send encrytped mail to.
When you want to encrypt something to someone the email client looks in its address book for the persons digital certificate that you are trying to send to.
That is why you see the error 'failed to find an encryption certificate for xxxxx'
Garry