Beginning with V8.12, sendmail distinguishes between running as a listening daemon (or queue processor), and running as a mail submission program (Section 2.6.2). This UseMSP option tells sendmail whether to run as a mail submission program. It looks like this:

O UseMSP=bool                configuration file (V8.12 and later) 
-OUseMSP=bool                command line (V8.12 and later) 
define(`confUSE_MSP',`bool') mc configuration (V8.12 and later) 

The bool is of type Boolean. If it is true, sendmail runs as a mail submission program. If it is false, or if the entire option is omitted, sendmail does not run as a mail submission program. In the default setup, the sendmail.cf file has this option undefined, and the submit.cf file has it defined. This option should never be defined in the sendmail.cf file.

When sendmail is run as a mail submission program, it runs under the uid of the user that ran it. If that user is root, and if the RunAsUser option (RunAsUser) was defined in the submit.cf file, sendmail becomes that user. Otherwise, it remains root.

One effect of defining this option to true is to allow group-writable queue files, but only if the group of the queue directory is the same as that of a set-group-id sendmail binary.

The UseMSP option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.