When capturing SMTP traffic with Network Monitor we are seeing data that does not belong such as added recipients or subjects.
All supported versions
Improper/dirty/bad traffic, specifically jumbo packets.
Network Monitor is designed to capture jumbo packets - but is configured to expect fewer of them in the network stream.
We can adjust settings under the Advanced Server Settings and recycle the services to apply the changes.
Once you change the settings and the monitor is stable you can check some new incidents and see if the issue persists. This should resolve garbled incident issues due to jumbo packets.
Defaults for above settings: