I see it as an exploitation
We can assume the sender is aware of SSL or encrypted mail and thus do not expect non-SSL mail to be private.
We can assume the sender is not aware of SSL or encrypted mail and thus does expect mail to be as private as they expect from physical deliveries.
Since we can't really assume all senders are security experts or setup with fully encrypted mail and transport protocols, we have to accept the second option; that the user is not aware and has an expectation of privacy.
The outcome is that sniffing that mail in transit exploits the ignorance of innocent civilians. It's a hostile act towards the person who's privacy is being invaded.
Granted, this then leads into the discussion about why all non-encrypted protocols should be abolished in favor of there existing encrypted version or replacement. We use SSH instead of Telnet for obvious reasons. We should be using smtps without acception between mail servers, pop3s/imaps/smtps from server to client and encrypted email. The real problem is getting mail admins to block non-encrypted transports and mail encryption that requires minimum effort by the user.
