The -t command-line switch causes sendmail to gather its list of recipients from the message's header in addition to gathering them from its command line. The -t switch takes no arguments.

When this switch is specified, sendmail gathers recipient names from the To:, Cc:, and Bcc: header lines. It also gathers recipient names from its command line if any were listed. Duplicates are discarded, the Bcc: header is stripped, and the message is delivered.

The -t switch is intended for use by MUAs. It should never be specified when sendmail is run in daemon mode.