Some users may get a bounce back indicating the account is suspended due to being inactive since a certain date.
If there are e-mails that are more than a year old lingering inside the inbox the account will be marked as inactive automatically.
This tends to happen more on accounts that have forwarding set up. If the account has forwarding set up, and does not leave a copy on the server, simply delete all existing e-mail on the server. This should bring the account back to active status within an hour or so.