What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
According to my checks, I found that the email addresses "firstname.lastname@example.org", "email@example.com", "firstname.lastname@example.org" have started to have failed records on their mail logs and all of them using the custom SMTP setting, so the problem could be related with your SMTP settings.
Please, check its credentials with using the guide from the below:
Alternatively, you can use our default email service provider to beat this issue. By setting the "Sender E-mail" option as "email@example.com" or "firstname.lastname@example.org", you can overcome this situation.
If the problem insists, please do let us know.