From the day, when you left ETH and you do not have any relations to the ETH anymore your email address will be available for 5 more month.
After that your mailbox will be deleted and your email address will not be reachable anymore.
If you are just transferred to an other ETH unit, all your email addresses are still valid.
In case your account is going to be expired, you will always be informed by email in advanced. If you think that the expiration is wrong or you have significant reasons why you should keep your account, contact us.
Please check the Leaving ETH chapter for all details.