Moving Mail to other accounts

Moving Mail to other accounts

I am wondering how to move the emails in one account to another account. For example, suppose we have an account for tom@c-ville.com and he has 300 emails in his account. If Tom leaves the company, and we create a new account for his replacement, Jill, under jill@c-ville.com, how do we get all of Tom's 300 old emails into Jill's new account so she can look them over and have access to them? In other words, how can I delete Tom's old account but retain access to his old emails, preferably in Jill's new account?