Ordinarily, when sendmail rebuilds an aliases database (as defined by the AliasFile option, AliasFile), it checks only the addresses to the left of the colon to make sure they all resolve to a delivery agent that has the F=A flag set (F=A). It is possible to also have addresses to the right of the colon checked for validity by setting the CheckAliases option to true.

The forms of the CheckAliases option are as follows:

O CheckAliases=bool             configuration file (V8.7 and later) 
-OCheckAliases=bool             command line (V8.7 and later) 
define(`confCHECK_ALIASES',True)    mc configuration (V8.7 and later) 
Onbool                          configuration file (deprecated) 
-onbool                         command line (deprecated) 
-on                             command line shorthand (V8.7 and later)

The bool is of type Boolean. If it is absent, the option defaults to true (do check the RHS of aliases). If the entire option declaration is missing, the default is false (don't check the RHS of aliases). The default for the mc configuration technique is false.

Addresses to the right of the colon are checked only to be sure they are good addresses. Each is processed by the canonify rule set 3 and then the parse rule set 0 to select a delivery agent. Processing merely needs to successfully select any non-#error delivery agent (see error). The sendmail program prints and logs the following warning and skips any address that fails to select a valid delivery agent:

address... bad address

If the address selects an #error delivery agent, the error text for that error is printed instead:

address... user address required

The CheckAliases option is further described in Section 12.5.2.

The CheckAliases option is not safe. If specified from the command line, it can cause sendmail to relinquish its special privileges.