Well, e mails are from a kinder, gentler time, and basically plain text. Unless you have encryption between every point between the you and your recipient, or the e mail itself is encrypted, its pretty trivial to sniff traffic and work out what's going on.
The standalone sysadmin has gone into depth on this, but in a nutshell, your e mails can be sniffed at the mail spools at either end, or in transit. Its not hugely likely that that specific sort of attack will happen unless its directed, or there's a massive compromise or government involvement at one of the ISPs involved. I'd guess that its something to be avoided if there's options but not a massive risk