Plesk Domain subscription was suspended due to the diskspace over usage, but the user continue using the mail service and disk space continue growing.
According to Administrator's Guide, Parallels Plesk Panel 11.5 mail service remains active after subscription suspending in Plesk 11.5:
The DNS and mail services are available for sites in suspended subscriptions, but cannot be managed by means of Panel.
This behavior will be considered and probably changed in future Plesk update that will be released after version 11.5. Here is a Feature Request without ETA:
1802704 - Disable mail service on domain if domain suspend, because diskspace continues growing
Create Event Handler in Parallels Plesk Panel 11.5 that disables mail service after subscription suspending. Remember to create reverse Handler that enables mail service after subscription activation.
In this KB article you may find example of configuration of Plesk Event Handler.