This is not the fault of Gravity Forms. Gravity Forms hands off email notifications to the wp_mail() function of WordPress. WordPress then calls the underlying mail delivery mechanism for your server, and every server is different. When you use an SMTP plugin, you eliminate all variability with host configuration and whatever mail transport they use.
If you install and properly configure an SMTP plugin, all mail from your WordPress site will be sent via that SMTP server, which is far more reliable than the web host's sendmail or other built in mail function.
There can also be issues with email receipt based on the recipient's email address and the address the email from your server appears to come from.
If you'd like help configuring the SMTP plugin to get more reliable delivery, we'll be happy to help.
If you'd like to read more about the problem, Joost de Valk has written something up which describes the problem and offers some solutions for making email delivery more reliable http://yoast.com/email-reliability/
Posted 8 years ago on Friday September 14, 2012 | Permalink