Whenever sendmail rebuilds the aliases database, it first clears the old database. It then rebuilds the database and, when done, adds the special entry @:@. Before sendmail attempts to use the database, it first looks in that database for the special entry @:@ that should be present. This curious entry is employed because it is always illegal in an aliases file. If sendmail doesn't find that entry (whether because a user ran newaliases or because another invocation of sendmail is currently rebuilding it), it waits 2 seconds for that entry to appear, then checks again. If the entry is still unavailable, the wait is doubled (up to a maximum wait of 60 seconds). The total time waited (after all the sleeps without success) is the interval specified by this AliasWait option.

When the @:@ appears, sendmail checks to see whether the database still needs to be rebuilt and rebuilds it if it does. If the special entry @:@ does not appear after the specified time, sendmail assumes that some other process died while that other process was rebuilding the database. This assumption paves the way for sendmail to go ahead and rebuild the database.

The forms of the AliasWait option are as follows:

O AliasWait=delay             configuration file (V8.7 and later) 
-OAliasWait=delay             command line (V8.7 and later) 
define(`confALIAS_WAIT',delay)    mc configuration (V8.7 and later) 
Oadelay                       configuration file (deprecated) 
-oadelay                      command line (deprecated) 

The delay argument is of type time and, if omitted, defaults to 5 minutes. If the entire AliasWait option is omitted or if delay is zero or nonnumeric, the database is not automatically rebuilt. If the unit of time desired is omitted, the delay defaults to minutes. If you use the mc configuration, the default for confALIAS_WAIT is 10 minutes.

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