One form of address is called a route address because it specifies a route (sequence of hosts) through which the message should be delivered. For example:

@hostA,@hostB:user@hostC

This address specifies that the message should first go to hostA, then from hostA to hostB, and finally from hostB to hostC for delivery to user.^[28]

^[28] Also see how route addresses are handled in rules in Section 19.3.3 and the F=d delivery agent flag in F=d.

RFC1123, in Section 5.3.3, specifies that delivery agents should always try to eliminate source routing when they are able. V8 sendmail takes an address such as this and checks to see whether it can connect to hostC directly. If it can, it rewrites the address like this:

user@hostC

This is called "pruning route addresses." There might be times when such pruning is inappropriate. Internal networks, for example, might be set up to encourage manual specification of a route through a high-speed network. If left to its own, sendmail always tosses the route and tries to connect directly.

The DontPruneRoutes option causes sendmail to never prune route addresses. The forms of this option are as follows:

O DontPruneRoutes=bool                       configuration file (V8.7 and later) 
-ODontPruneRoutes=bool                       command line (V8.7 and later) 
define(`confDONT_PRUNE_ROUTES',`bool')       mc configuration (V8.7 and later) 
ORbool                                       configuration file (deprecated) 
-oRbool                                      command line (deprecated) 

The argument bool is of type Boolean. If it is missing, the default value is true (nothing special is done with route addresses). If the entire R option is missing, the default becomes false (route addresses are pruned). With the mc configuration technique the default is false.

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