Another clue!
Late last night, one of the 6 Airsight Cameras sent a motion detected email!
Upon receipt of that email, I used my iPad to connect to the camera both via the Airsight App and directly via Safari and both initiated a successful test of the email configuration.
This morning, with no changes made to the Airsight Camera, I repeated the email configuration test of that camera via Safari, the Airsighjt App and my Desktop and all failed.
So, could the issue be a time-of-day issue and if so, what controls this?