The camera can be configured to send snapshots/video via SMB/FTP/email/SD. The camera can only handle so many events at once. When the camera is still busy processing an event (ie sending a video clip via FTP) while another motion detection event occurs, an email will be sent rather than an email with the snapshot/video. The purpose is to at least send something to notify of all events even if the camera cannot handle all the processes at once.
If there are several motion events in succession, logs may be sent in lieu because the camera is still processing the first few video clips
If several events are configured SD recording, FTP and email video, it is most definite email logs will be sent
If someone configures video/snapshow motion to either SD, email, FTP or SMB and motion detection is triggered, an email log should rarely ever be sent. If so, lower the motion detection sensitivity and increase the motion detection delay so the camera has time to actually process and send the event before it needs to worry about the next event. In other words it someone walks by and is there for 5 minutes, the camera could be processing 100+ events. If this is the case in that environment, an NVR (network video recorder) would be much more practical especially if there are more than 2-3 cameras in one location.