If an account has not been checked in 90 days, it is considered inactive and will be locked. Any messages sent to that address will be returned as undeliverable.
If an account has not been checked in 120 days, all e-mails for the account will be deleted. Any messages sent to an inactive address will be returned as undeliverable.
To "reactivate" the account, the user will have to either pop their mail, or login through Webmail. This will not recover any deleted or undeliverable mail, but will allow new mail to be delivered. Note that this does not prevent the account from being locked in the future. Should the account be inactive for another 90 days, it will be locked again.