My day job includes working on FreeBSD systems and also doing open source FreeBSD work (push upstream, pull down to us). There are sometimes embargoed security notices in my email. There is no chance I will permit my employer to MITM my SSL and risk some clowns in corporate IT from obtaining these mails. (highest security ones are GPG encrypted, but others are not)

I need my personal email to do my work. It needs to stay secure from even my own employer. Period.