The F=I delivery agent flag tells the local sendmail to send the following VERB^[31] SMTP command to the receiving sendmail:

^[31] VERB is defined in RFC1700.

VERB                               ours sends
200 2.0.0 Verbose mode             recipient replies

The VERB SMTP command causes the receiving sendmail to go into verbose mode and to set its deliver mode to interactive. This has the same effect as would occur if the receiving sendmail had been run with the command-line options -v and -odi set.

The F=I delivery agent flag is intended as an aid in debugging a remote receiving site's sendmail. The VERB SMTP command causes that remote site to run in verbose mode. By temporarily adding the F=I delivery agent flag to a delivery agent's definition and then running sendmail locally with the -v command-line argument, you can watch both the local and the remote's verbose output. Each line of the remote site's verbose output will be seen locally, prefixed with 050.

Note that if the PrivacyOptions option on the remote site's sendmail is set to noexpn (See this section) or noverb (See this section), that site's response to the VERB SMTP command will be this rejection:

502 5.7.0 Verbose unavailable

Also note that if the other side is not running sendmail, you might see other errors:

501 No argument given in VERB command.   PMDF V5.0
250 Ok                      post.office v1.9.1 (but ignores the request)

In both these cases, the request to the other machine to go into verbose mode has failed.