New users to Exchange (or AD) more than often are confused with the above recipient types. Below is a brief explanation.
- Mailbox-enabled User: full functional object in AD/Exchange. Can send/receive email using local Exchange org, has SID and access to domain resources, and has mailbox on local Exchange server
- Mail-enabled User: has external email address only so can send/receive email only externally. Can be seen in GAL. Has SID and access to domain resources
- Mail-enalbed Contact (or just Contact): Same as Mail-enabled User but lacks of SID therefore doesn't have access to domain resources
- DL: A security group or distribution group with email address. If it's for email purpose only, make it a distribution group.
This is a very basic knowledge for whoever works on Exchange/AD, but I was amazed by the number of people who didn't know.