About the stopping thing I agree it sounds rather harsh, but OTOH Linux is too powerfull to carry on w/o serious RTM'ing.
Maybe hop on over to cert.org and enter "sendmail" in the searchbox and see what vulnerabilities surface so you get a general idea. Search for the "Unix checklist". Now hop on over to linuxdoc.org and search for "Securing Optimizing Linux" or use Tricky's siglink
Btw loglevel in sendmail is:
0 No logging.
1 Serious system failures and potential security problems.
2 Lost communications (network problems) and protocol failures.
3 Other serious failures.
4 Minor failures.
5 Message collection statistics.
6 Creation of error messages, VRFY and EXPN commands.
7 Delivery failures (host or user unknown, etc.).
8 Successful deliveries and alias database rebuilds.
9 Messages being deferred (due to a host being down, etc.).
10 Database expansion (alias, forward, and userdb lookups).
20 Logs attempts to run locked queue files. These are not errors, but can be useful to note if your queue appears to be clogged.
30 Lost locks (only if using lockf instead of flock).
Also have a look if you need to bump up the level in syslog.conf (man syslog.conf).
Btw, you say all is working fine, but *how* do you know/check that?.. :-]