The sendmail package that comes with Slackware has no LDAP support. In case that you need it then this article might help. It covers the following topics:
1. Recompile and reinstall sendmail with LDAP support
2. Reconfigure OpenLDAP
3. Reconfigure sendmail and import aliases into LDAP
Before you try to use this description make sure that you know a little about OpenLDAP and sendmail itself! This article was tested on a Slackware environment but should also work for any other Unix/Linux (except for recompiling the sendmail package).