The sendmail program can be compiled to use the user database (userdb) by defining USERDB in the Makefile (USERDB). If an address is selected by the parse rule set 0 for delivery by a delivery agent with the F=l flag set, and if it remains unaliased even if the F=A flag is set and if the F=5 (F=5) delivery agent flag is set, it is looked up in the user database. The -d28.1 (a.k.a. -d28) debugging switch is used to watch the interaction between sendmail and the user database:

udbexpand(addr)

Here, addr is the address being looked up.

The sender is looked up in a similar fashion. The intent in this case is to correct information such as the return address:

udbmatch(login, what) 

Here, login is the login name of the sender and what is the mailname for sender lookups. If the lookup is via hesiod, sendmail will print the same information, like this:

hes_udb_get(login, what) 

If the sender is found in the database, sendmail prints:

udbmatch =  => login@defaulthost 

Here, login can be a new login name. The defaulthost is either the sitewide host for all reply mail as defined in the user database, or the default destination host for a particular user.

In the event that a db(3)-style user database fails to open, the -d28.1 debugging switch displays the following error message:

dbopen(database): reason for failure here