Ok, thanks for the clarification. That definitely shouldn't be happening. Unfortunately, I don't have a quick fix, as the email sending system is managed by our web host, not us directly.
However, in a few days, I will investigate further in this problem, and file a support ticket with our web host if necessary. I'll post an update here. In the meantime, if anybody else is having the problem, please post in this thread--it's more data points if I need to file a support ticket.
Thanks!