JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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.
This usually happens when the email service provider notices an unusual email traffic, let's say your client's email is receiving multiple emails from email@example.com, that will trigger his provider that that the source is sending spams. What he can do is that he can add the "sender email" in his/her contact list or whitelist us instead, depending on the options the email client has.
You can also try resolving this using a custom Sender Email or SMTP:
I hope that helps.