Thanks again for your continued help.
I did try that before posting to this forum, and did it again this morning after seeing your response and suggestion. The results are the same, no email is received when only 1 field is sent. If 2 fields are sent, then email is received. The behavior I described is 100% reproducible.
I tried it originally with my own domain email, and the alternate email provider I used for comparison is Hotmail.
For example, I used the following, which resulted in no email being received (I used my email address of course intead of "myname").:
[contact-form firstname.lastname@example.org'][contact-field label='Email' type='email' required='1'/][/contact-form]
Do you have any other suggestions, debugging steps, or other information?