How secure is email?

I heard a saying: "Never put something in an email you wouldn't put on a postcard".

Is there any truth to this?

Somebody could be sniffing your packets _{or panties}!

Also lots of newfags access email without encryption, meaning if they use one compromised network, an attacker can access their email inbox (and possibly read your message)

There is strict legislation for mail, including postcards, but AFAIK not so much around email.

Use PGP.

This is true. Sending the email as "plaintext" is equivalent to writing your message on a postcard, anybody who can see the postcard will be able to see the message. If you don't know how to use encryption properly (look for GPG), you should never put something in an email you wouldn't put on a postcard.