If your company or university uses LDAP — Lightweight Directory Access Protocol — for a centralized telephone directory, your desktop email client can most likely connect to it. However, if you want to use your iPhone to delve into the same telephone directory, you can't. At least not unless you have Boneware's LDAPeople.
LDAP is a standard for indexing names, addresses and other bits of information so you can search for that information in a variety of ways using plain English. For example, if you wanted to reach someone in accounting at your company but you don't have a name, you might use LDAP to hunt for them by title.
LDAPeople is an iPhone client that enables you to search LDAP telephone directories, send emails to people in the directory, make phone calls to them, look up their addresses and so on.
Once you have the contact information, you can export it to your iPhone Contacts app. You can also use the contact's address to locate them in Google Map.
It's the only LDAP solution in the App Store I could find. If you need to access LDAP, this is what you're looking for.