Here is what I received from my hosting company’s senior tech regarding problems I’ve had with mail delivery:
It looks like your FROM (name and email) are dynamic based on what the user on your website inputs, is this correct?
Doing it this way will cause the messages to bounce back, depending on DMARC/SPF and SMTP requirements. It will go through FROM some addresses, and bounce back FROM others. Which I think matches the description of your issue. The best solution is to have the FROM address set statically to something @ your domain name, like firstname.lastname@example.org (or info@ etc.) this will guarantee the message to be delivered.
What you have configured currently is calling “forging the from address” which is not supported in most cases. Its a common practice of spammers so most SMTP servers on the internet forcibly bounce anything with forged information.
You can read more about it at the below links, and why its disabled or discouraged.
What’s really going on with Rapidweaver’s contact form? Is it really forging the from address?