Make sure your web server is whitelisted on your email server if you're using UserDefinedForm with the 'From' address of the person who filled in the form.
It's great that UserDefinedForm can generate an email with the 'From' address of the person who filled in the form, because the recipient just needs to hit 'Reply'. Otherwise the recipient needs to copy and paste from the email address field in the body of the generated email, which they often forget to do.
Trouble is, if the domain in the 'From' address has SPF configured, then the recipient's mail server will probably detect it as spam, because the email originates from the web server, not the authorised SMTP server for that domain.
The obvious solution is to whitelist the web server. Unfortunately, this is often not possible, or their network manager (how can I put this politely?) doesn't understand or is unco-operative.
So it would be really useful if, on the UserDefinedForm's 'Email Recipients' tab, you could specify a 'Reply-to' address in the same way as the 'From' address. That would get it past the spam filters.