Can JHED be used as an LDAP Directory lookup within my e-mail client?

Yes, most e-mail clients will allow you to configure a Directory Server profile to search for addresses.  The settings are as follows:

LDAP host/Server Name: ldap.johnshopkins.edu
Port: 636
SSL Enabled: True
Search BASE DN: ou=people,dc=win,dc=ad,dc=jhu,dc=edu
Username: cn=<JHEDID>,ou=people,dc=win,dc=ad,dc=jhu,dc=edu
Password: JHED/WIN Password

NOTE: This is only available on the campus network (either physically on site or logged on to the VPN)