Can another PC access my emails?

Picture this:
I’ve got 2 PC’s:
One of them is right in front of me and I can access my emails at any time on that PC. The other one is approximately 10 km away from me, on a different address, with a different IP and I can’t access it. Still I’ve got the feeling that the second one can access my emails.

1 How can I check whether the second PC can actually access my mail?

2 How can I make sure that the second PC cannot access my mail any more?