Why emails from the contact form do not arrive
Most often because the site sends the message from an address in your domain but from somebody else's server, so the recipient treats it as impersonation.
Email has three safeguards. SPF states which servers may send on behalf of the domain, DKIM signs the message, DMARC defines what to do when something does not add up. A form wired up in a hurry usually meets none of them and lands in spam or disappears without a trace.
The correct solution is sending through a transactional email service, with the DNS records filled in, and saving every submission on the server. Then even if the mail fails, the enquiry is not lost. It is also worth sending yourself a test submission once a quarter and checking that it arrived.
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