:: If it weren't - that is so easy and cheap to intercept that you're just asking for someone to please take your passwords and steal your identity. These days unencrypted connections from your client to your SMTP/IMAP/POP3 server are practically unheard of.

You see in our diagram that one of the SMTP connections is supposed to be encrypted (this is the one that would be "LAN" traffic) and the IMAP connection as well (this one is either LAN-like traffic or is connecting to localhost, which is a different scenario altogether).