F-SECURE ANTI-VIRUS FOR MICROSOFT EXCHANGE 9.00 Administrator's Manual - page 196
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3.8.2
Network Configuration
The mail direction is based on the Internal domains and Internal SMTP
senders settings and it is determined as follows:
1.
E-mail messages are considered internal if they come from internal
SMTP sender hosts and mail recipients belong to one of the specified
internal domains (internal recipients).
2.
E-mail messages are considered outbound if they come from
internal SMTP sender hosts and mail recipients do not belong to the
specified internal domains (external recipients).
3.
E-mail messages that come from hosts that are not defined as
internal SMTP sender hosts are considered inbound.
4.
E-mail messages submitted via MAPI or Pickup Folder are treated as
if they are sent from the internal SMTP sender host.
If e-mail messages come from internal SMTP sender hosts and
contain both internal and external recipients, messages are split
and processed as internal and outbound respectively.