I almost noted something similar, but was hoping what I wrote would be enough to improve your outcomes. The Rapidweaver contact form does not support SMTP. You’ll need to use a stack. There are many that will work.
SMTP uses a valid email address to login and send a given email. PHP’s mailer function does not. The address might exist. It might not.
SMTP has additional benefits. Email can be sent across the web encrypted…It’s then decrypted by receiving mail servers. That’s not possible with your host’s PHP mailer.
When you update your form to use SMTP, you should add a DKIM record(s) in addition to your SPF record. Your mail provider will tell you what to add through their help documentation.