I have an OID from A MIB Compiled and distributed from: http://www.wtcs.org/snmp4tpc/
The mib contains OID's for Exchange 5.5 and an example would be:
OID: 1.3.6.1.4.1.311.1.1.3.1.1.1.6
which is equivialant to mtaadminconnections. i can use SNMP MIB scout and correctly pull a value from the Queried Exchange Server after installing the mib on that exchange server without installing the mib on the management station. Even after installing the mib locally i still get message: unknown variable (OID)
Probably you have enabled "Automatically append the single instance sub-identifier" option (located on Miscellaneous page in the Options dialog). Disable it.