This message appears when I choose to "show images" in a message. The YES button start a process to get the certificate, (knowing nothing about it, is on not is it real or fake?), and to import in your address book.
This is descouraging for some customer, that are unbale to understand what it happening.
I think there would be some buttons as "allow trust for this message/session only" and "allow trust sender" or other, this way user may not interact with other dialog difficult to understand.