Smtp is a plain text protocol .  What does that mean?  It means the actual emails you send are about as private as a postcard.  The sender, the recipient, and the message itself are all plainly visible to anyone who comes in contact with it. Chances are that your letter carrier and the the folks at the post office don’t care about what you wrote on your post card, and are too busy to bother reading it.  Likewise, most of your neighbors are not snooping about when you drop your postcard in the mail box or when the letter carrier picks it up.  Still, i wouldn’t write anything in an email, that i wouldn’t write on a postcard.

Further Reading

E-mail – Wikipedia article

How Email Works – HSW article

RFC 822 – The technical standard