Beginning with V8.7 sendmail, addresses with and without a host part that resolve to local delivery are handled in the same way. For example, user and user@local.host are both looked up with the User Database (userdb) and processed by the localaddr rule set 5 (Section 19.6). This processing can result in those addresses being forwarded to other machines.

With the stickyhost feature, you can change this behavior:

FEATURE(`stickyhost')

By defining stickyhost, you are telling sendmail to mark addresses that have a local host part as "sticky":

user            not sticky
user@local.host sticky

Sticky hosts tend to be delivered on the local machine. That is, they are not looked up with the User Database and are not processed by the localaddr rule set 5.

One use for this feature is to create a domainwide namespace. In it, all addresses without a host part will be forwarded to a central mail server. Those with a local host part will remain on the local machine and be delivered in the usual local way.

Note that this is opposite the behavior of the former notsticky feature of V8.6.