Here is my client’s testimonial:
I changed my business email firstname.lastname@example.org (the one configured to receive the form submissions) to G Suite. From this moment on, the form messages started to be directed to my Spam Box and after a few days stopped being received at all. I do receive email from other senders, though.
I erased all MX records from my hosting panel to make sure there wasn’t trouble with G Suite.
I called Google and I was told it was a development issue, that I should edit the IMAP outbox so it would recognize Google as the way out.
I called the hosting provider. They accessed the website and made some tests on the forms. I was told to way 24h but nothing happened. I called again and they couldn’t solve it.
I talked to the designer who made the website and she made a test: she changed the form subscription email to email@example.com (which is also set as G Suite) and I started receiving the submissions.