WPForms is a good plugin, and most "not sending email" advice for it is the same generic SMTP checklist you have already read. So let's skip that and start with the three failures that are specific to WPForms, all of which share one property: the visitor sees a success message, and you receive nothing.
Failure 1: the entry was marked as spam — so no email is sent
This is the one that surprises people, and it is documented behaviour. From WPForms' own documentation on spam entries:
"After enabling this option, the form will allow all submissions (including spam entries) instead of blocking them. However, form notification emails won't be delivered if an entry is identified as spam."wpforms.com — Viewing and Managing Spam Entries
Read the consequence carefully. A real customer fills in your client's form. Akismet or the built-in anti-spam decides, on some heuristic, that they are a bot. The customer sees "Thanks for contacting us!" and goes about their day, satisfied. The entry lands in a Spam folder inside WordPress. No notification is sent to anyone. Nobody knows this happened unless someone thinks to open the spam tab.
