Make sure the time and date are correct on that PC. That error can be caused by the time being off by more than a few hours.
Thanks for your reply.
My computer was off by exactly 1 hour.
What is strange that only two of my contacts were being blocked.
I just sent both of these contacts test email messages to see if fixing my clock solved the problem.
This did not solve the problem as both of the email messages I just sent were blocked.