The user mail service is defined in a Class-of-Service definition called standardUserMail:
# # Definition for user mail service bundle # dn: cn=standardUserMail,<ugldapbasedn> changetype: add objectclass: top objectclass: LDAPsubentry objectclass: extensibleObject objectclass: cosSuperDefinition objectclass: cosClassicDefinition cosTemplateDn: o=cosTemplates,<ugldapbasedn> cosSpecifier: inetCos cosAttribute: mailAllowedServiceAccess cosAttribute: mailMsgMaxBlocks cosAttribute: mailquota cosAttribute: mailmsgquota daServiceType: mail user NOTE: When the Delegated Administrator configuration program installs the standardUserMail definition in the directory, the variable <ugldapbasedn>, shown above, is replaced by your root suffix (such as o=usergroup).
The daServiceType attribute defines this as a mail service for users.