To beat the old drum: Email isn't intended to be secure anyway. Relying on email addresses to maintain privacy and authenticity is like relying on Caller ID to verify callers' identities. (See spoofcard.com.)
Encrypt, encrypt, encrypt. Or, encrypt.
Encrypt, encrypt, encrypt. Or, encrypt.
I guess a normal level of paranoia hasn't quite reached those companies yet huh?
And you can't control who sends you email.