Besides using the `-D' flag for recvmail and postmail, and all the usual sendmail debug options, you can get recvmail to save copies of mail messages for later perusal.
If the directory /var/spool/post/headers exists, recvmail will save a copy of the headers of every mail message to this directory. If the directory /var/spool/post/messages exists, recvmail will save a copy of the full text of every mail message to this directory. The daily script run by cron will clean these directories out; if, however, the directory /var/spool/post/yesterday exists, it will instead clean out that directory and move the files in messages and headers to that directory.
The saved files will have names of the form YYYYMMDD_HHMMSS_N where YYYY is the year, MM the month, DD the day, HHMMSS the time, and N is the original spool file name.