Watchguard V10 User Manual page 319

Description
The proxy action should be used with the following
services
General tab
This tab allows you to specify general values for Incoming
SMTP content filtering.
Maximum Recipients
Firebox Vclass User Guide
A description of the proxy, for your reference.
The default service for the SMTP proxy is TCP
Ports 25. This section is informational only. The
proxy will filter all content of the specified type,
regardless of the port used.
Specifies the maximum number of email recipients
to which a message can be sent. This acts as a
counter, and allows the specified number through,
then drops the remaining addresses. For example,
Proxy Parameters Reference
287