Form is not sending autoresponder

My forms just stopped sending autoresponders, while the notification email still works. I have tried to test different emails and the problem persisted.

After sending a couple of test email, the form started working again, but a few autoresponders got lost. How can I avoid this happening again?

I inspected the forms and the auto-responders of the form CPSWeek Student Travel Support Application  didn't have a sender email:

This can make the email providers block the email.

I made a test submission on the form 60452672991361 and received the auto-responders:

Let us know if the problem persists.