The risk isn't that much that your employer gaining access to your email (though you may potentially be risking the contents of emails that you view from that machine getting saved and accessed by someone at the company). It's more that you've legally entangled things. If your employer is sued or investigated, a judge can issue a subpoena for them to turn over records. If those records show that employees accessed external accounts from work systems, now they can get a subpoena to access those accounts and any other devices that have accessed those. I've seen this happen to friends. Employer gets sued and as part of discovery, they had to hand over all of their personal devices because they hadn't kept church and state separate. Took them many months and significant legal expenses to get their stuff back. If you never access personal stuff from your work devices and never access work stuff from your personal devices, you'll never be in that position.