In the System Config area, under "Configure an SMTP Filter" you can set limits on message delivery.
SMTP filtering provides for rule-based specification of filters that are applied to messages as they are received. Filters may contain multi-faceted triggering based on a combination of source, destination, subject or message body contents. When a filter matches the contents of a message, one or more elements in a series of actions is invoked. Actions include message discard with logging, message forwarding to an alternate address and copying of a message with delivery.
Filters are constructed using standard UNIX regular expression processing, or by simply using keywords. This makes Post.Office SPAM filtering extremely powerful.
For detailed documentation on regular expressions there are many resources on the web:
Note: Not all functions in those documents are supported.