Me and DH share an email address, he is self employed and works from home and it is also the business email address.
Nothing sinister, we just share a laptop and I just cannot be bothered logging out of one email to check the other (we had the same email host, couldn't both be logged in at the same time on the same computer).
I suppose it's more me using his email for ease than actually sharing one though. I still have my own, I just don't use it.
He doesn't seem bothered by it at all. I'm sure if he wanted to cheat he would use FB, or Whatsapp or something, or text messages.
I've been in relationships with no trust in the past, one didn't trust me at all, accused me of all sorts with no reason to except his own paranoia. The other was constantly flirting online with other women, hoping something better would come along, it nearly broke me.
Never again will I be in a relationship without complete trust. It's just not worth the misery.