Yes, that would be no issue. Pop3 c.q. port 110 if that's the port you're speaking about, is not used by MTA's, only by mail clients from customers to pick up their mail.
So you should be fine closing it and whitelisting the client for it. Or try to persuade the client to use pop3s, so pop3 with SSL on port 995, this way you can also close port 110.