I had the same issue and this works out.
1. Go on the MailPoet settings.
2. Then Advanced, and at the bottom find “Reinstall from Scratch” and hit the Reinstall button.
3. Just follow the steps in there and make sure the email that is sending from on the first step is from your domain and not a gmail, yahoo, etc.
4. Then you need to create your forms again.
Hope it works for you.