[rmc]

Their email client can do it automatically. Basically, you just need to tell them, "Official emails will always have a big, green border around them."

You then have 2 problems: (a) What email clients will support it and (b) con artists will just put big green borders around their spam emails.

[betterunix]

Further proof that HTML mail is a terrible idea...

(Edit: It is also conceivable that a client could put another prominent border around HTML mail, to mitigate the issue somewhat.)

[rmc]

Heh, and you think people will be able to tell the difference between HTML email with a border, and the border around certified email.