self-decrypting message

[listo factor]

> ...Can I send an encrypted e-mail so that it decodes itself automatically once it reaches the recipient?

An e-mail message is just a piece of data; it is always a
computer program (i.e., a piece of software, not data) that
performs either encryption or decryption. It is therefore
not possible to construct a message that would somehow
"decrypt itself" upon arrival on ~any~ computer, but it is
possible to set up a program on a specific recipient's
computer to do that. Details would depend on the nature
of recipient's computer and software - specifically the
e-mail client program - used on it. This would of course
only make sense if the attacker has absolutely no access
to recipient's computer, not even via the network.