The preferred way to run mail.local is in LMTP mode. LMTP is described in RFC2033.

Without LMTP (when there are multiple users in the envelope) it is possible for delivery to fail for a single user. When this happens, unexpected problems might occur with the good users. Sometimes they will receive duplicate messages and sometimes they will receive mail after a long and unexplained delay.

At sites that handle a large amount of mail for many users, LMTP mode is highly recommended. Multiple recipients per envelope are gracefully handled with LMTP. Each hands a separate error or success code back to sendmail, so there is never any confusion about what was and was not delivered.

If, despite these advantages, you wish to turn off LMTP mode in mail.local you can do so by omitting this -l command-line switch:

define(`LOCAL_MAILER_ARGS', `mail.local   DOL(u)')
                                        
            -l has been omitted

Here, we first omitted the local_lmtp feature to prevent the local delivery agent's flags from being set up for LMTP. We then declare mail.local without the -l to prevent it from speaking LMTP. Finally, we add $u (with the DOL macro) to cause the list of recipients to be passed in the command line.

See Section 5.3.2 for a description of how to install mail.local for use with the preferred LMTP mode.