Ensuring users’ email addresses are kept up to date

My site needs users to register with email addresses that are not freely available, like company or university email addresses. When users register, they get sent an email containing a link to click on so that we know the address is valid at that time. However, after a while, people are going to move companies, leave university etc. I would like to ensure that these email addresses are kept up to date. How can I do that whilst being as least annoying as I can? (eg every X months I could send an email saying please verify this is still your email address, but it seems fraught with danger.)