Ordinarily, when mail is delivered using LMTP, the LMTP conversation begins like this:

220 yourhost LMTP ready             mail.local sends
>>> LHLO yourhost.your.domain       sendmail sends
250-yourhost                        mail.local sends
250-8BITMIME                        note
250-ENHANCEDSTATUSCODES
250 PIPELINING

Here, mail.local is telling sendmail (in the fourth line) that it can correctly handle 8-bit MIME (EightBitMode) in received messages. If your site is a 7-bit only site, you should not allow mail.local to accept 8-bit MIME messages. You disallow 8-bit MIME by specifying this -7 command-line switch:

FEATURE(`local_lmtp')
define(`LOCAL_MAILER_ARGS', `mail.local -l -7')

Here, the -7 command-line switch causes mail.local to exclude -8BITMIME from the list of features it supports.

220 yourhost LMTP ready             mail.local sends
>>> LHLO yourhost.your.domain       sendmail sends
250-yourhost                        mail.local sends
250-ENHANCEDSTATUSCODES
250 PIPELINING