We’re not website designers or programmers, but we do understand emails systems and the anti spam systems that are now needed. There is a lot of spam out there, some just marketing, and some nasty and carrying fraudulent
We have one client who has a Word Press contact form on their website. A potential client can fill out the form, with their name, email address and a short message. They have an anti “bot” addon too. When they hit submit, an email is generated to go to my clients email address.
The source of that email is the email address that the new client has entered. The problem is, the email DID NOT COME FROM THE SERVERS THAT MAINTAIN THAT EMAIL SYSTEM. The server than generated is the where the website is hosted.
Any email system with a degree of spam control checks the address of an email, and where it has come from. It then checks where is SHOULD HAVE COME FROM. If it matches, legit email. If it does not, SPAM or JUNK.
The solution would to send the form from a fixed address, one of your own. You also need to send it using the correct SMTP servers. It has to come from the actual server it is meant to come from. Office 365 email from Microsoft servers, Gmail from gmail servers etc.
The link below might help with the technical aspect of what we said above.