Yes, in the way your snail mail is exposed to postal trucks, mail handling agents and office clerks.

Not when the snail mail is in an envelope. When you send plaintext email, you are exposing the entire body of the message to casual inspection by any of the mail servers that are involved in delivering it. Unlike the postal system, however, your email is copied and stored by the servers involved in sending it, and you have little control over how long it is stored.

It would be as though you communicated by sending post cards, and the postal works took every post card and ran it through a copy machine, leaving them all in a neatly organized (by sender and receiver) pile somewhere.

SMTP does not inspect the body, it's outside of scope of the protocol.

Also, unsealing an envelope on a hot surface is trivial. Nitpicking aside, both mediums are not inherently secure.