Issues with Autoresponder Email Confirmation - emails not being sent randomly

I have a form with an autoresponder email to be sent to the person who fills out the form. There are times when the auto-email simply does not send.

This morning 2 people filled out the form within 30 minutes of each other. One person got the email, the other did not. We have about 20 people who fill out this form regularly---different people have issues on different days. There seems to be no pattern as to who does not get the email or when. This has been an intermittent issue for several months.

Note: the emails are not going into spam. And the submissions are going through, it's just the auto-emails are not.

Please make sure that the field "Opened By Email Address" is not part of any conditions, because the conditions may overrule the settings of the autoresponder. You should also not use encrypted forms. The autoresponders will not work with any encrypted forms.