The -d27.1 (a.k.a. -d27) debugging switch causes sendmail to print each step it takes when processing local addresses through aliasing. First, sendmail prints the addresses being aliased:

alias(addr)

Here, addr is the address (usually a local username) that is about to be aliased. Note that it can already be the result of previous aliasing. If the addr can be aliased, its transformation is printed as:

addr (host, user) aliased to newaddr

Here, addr is the address before aliasing, and the newaddr is the new address that resulted from successful aliasing. The host and user are the hostname and username from the recipient part of the envelope. If the addr cannot be aliased, nothing is printed.

During initialization, if the aliases database cannot be opened, the -d27.1 debugging switch causes sendmail to print:

Can't open aliasfile

Here, aliasfile is the full pathname of the aliases(5) file, as declared by the AliasFile option (Section 24.9) or implied with the service-switch file set by the ServiceSwitchFile option (ServiceSwitchFile).

If the failure was due to a faulty map declaration, sendmail logs the following error:

setalias: unknown alias class dbtype

If the map is not allowed to provide alias services, sendmail logs this error:

setalias: map class dbtype can't handle aliases

If sendmail is trying to create a database file and it can't (usually when it is run with the -bi command-line switch or run as newaliases), the -d27.1 debugging switch causes the following error to be printed:

Can't create database for filename: reason here

A self-destructive alias can cause a dangerous loop to occur. For example, the following two aliases can lead to a loop on the host mailhost:

jake:           Jake_Bair
Jake_Bair:      jake@mailhost

The -d27.1 debugging switch causes the following message to be printed when sendmail tests an address to see whether it loops:

self_reference(addr)
        ... no self ref                if it didn't loop
        ... cannot break loop for "addr"if it's unbreakable 

An alias loop is unbreakable if no local username can be found in the list of aliases.

The -d27.1 debugging switch also causes sendmail to print a warning if it cannot open an alias file for rebuilding (AutoRebuildAliases, the AutoRebuildAliases option):

Can't open file:  reason here
newaliases: cannot open file:  reason here

Here, the error might be caused by the file simply not existing (as would be the case if it was NFS-mounted on a down host) or an I/O error (as would be the case if it was a bad disk):

warning: cannot lock file:  reason here

Failure to lock can be caused by system errors or by the file being read-only. Note that maintaining an aliases file under revision control can cause a read-only copy to exist, resulting in the following error:

Can't create database for file:  reason here
Cannot create database for alias file file

This error indicates that the output file (the dbm(3) or db(3) file) could not be created or written.

The -d27.1 debugging switch also causes sendmail to print the following message when it is attempting to read the user's ~/.forward file:

forward(user)

If the user has no home directory listed in the passwd(5) file, sendmail issues the following message with a syslog(3) level of LOG_CRIT:

forward: no home