If you use the Gravity Forms WordPress plugin, be sure you don’t set it to send mail “from” the e-mail address of the person filling out the form. If you do, you’ll have trouble due to recent “DMARC” anti-forgery changes some companies (including AOL and Yahoo) have made.
To avoid problems, make sure that Gravity Forms (and other such forms) send mail “from” the Web site domain name the form uses. For instance, if your Web site is at www.example.com, you could send mail from “email@example.com”. Here’s a helpful page that explains how to properly set up the Gravity Forms address with DMARC in mind.
By the way, this is just a specific case of the general rule of “don’t send mail from addresses you don’t own”. The simple way to think of it is that you’re not (say) AOL or Yahoo, so your Web site shouldn’t send mail claiming it’s from aol.com or yahoo.com addresses. AOL and Yahoo don’t want other people doing that. Always send mail only from your own domain name.