Explanation:
The SMTPSaveOutboundToFile setting saves a copy of outbound SMPT messages to text files for diagnostic purposes. These messages will be saved as individual TMP files within the TMP directory specified for the server. The file names will be STXXXXXX.TMP (where XXXXXX will be 6 numerical digits). The contents of the files will contain the header information and the MIME information.

Default:
When this option is absent or set to 0, the associated behavior is disabled.

Recommendation:
Remove this setting or set the value to 0. Saving outbound SMTP messages to temporary files has a negative impact on server performance.

Restarts:
A server restart is required for the change to take effect.

References:
Technote 1088993
Trapping outbound SMTP MIME information
http://www.ibm.com/support/docview.wss?uid=swg21088993

Domino Configuration Tuner:
This setting is included in the DCT catalog of best practices.