Debug flags for sendmail

Common flags:

-d0     General debugging.
-d1     Show send information.
-d2     End with finis( ).
-d3     Print the load average.
-d4     Enough disk space.
-d5     Show events.
-d6     Show failed mail.
-d7     The queue file name.
-d8     DNS name resolution.
-d9     Trace RFC1413 queries.
-d9.1   Make host name canonical.
-d10    Show recipient delivery.
-d11    Trace delivery.
-d12    Show mapping of relative host.
-d13    Show delivery.
-d14    Show header field commas.
-d15    Show network get request activity.
-d16    Outgoing connections.
-d17    List MX hosts.

-d12     Set flag 12 to level 1
-d12.3   Set flag 12 to level 3
-d3-17   Set flags 3 through 17 to level 1
-d3-17.4 Set flags 3 through 17 to level 4

More details in these following links...

Geocities, Monkeytools? , Sendmail 2nd edition, Whole scoop on configuration file