You see the following warning in the log file:
WARNING: [5050-24-398-00000000] Unable to find matching NM_SCHED_ID email@example.com
in organizer 040000008200E00074C5B7101A82E0080000000030E8523E6D11CC01000000000000000010000000A70A8BC7561E4A46BF5E220EDE38D901 calendar
NME looks for the scheduled id in the target mailbox for the organizer to see if the meeting already exists so that NME can link it up correctly and replies work. If this meeting was scheduled only between Notes users then it will not find it. This search only will return a value if the meeting was scheduled by an Exchange user and sent to Notes
This is one of the warnings that are safe to ignore.