A contact form has been created on a Contao CMS (v4.9.20) at example.com. When setting the Recipient address of the contact form to be received at an external email address different from the primary domain (example.com), such as example@gmail.com, the contact form is received at the gmail account. When the Recipient address is set to an internal email account, such as info@example.com, the web host sends an error, "Mail delivery failed: returning message to sender" and indicates no such user. This user has been established and the MX for the account has been working without problems.

It so happens that, the System Administrator email has been set to my external email account, thus I receive these Mail delivery messages; however, if I set the System Administrator account to an internal email account, those messages are not received, but the problem is not fixed. Contact form submissions are still not received nor are system administrator messages.

My thought is this is probably not an issue with Contao, but rather a configuration issue with the web host. I am looking to state my case for support from the web host, but I can't identify the script that is sending emails from the CMS or what method Contao uses to send emails. I would assume that I could create a separate PHP test page to simulate the issue as I'm assuming that the CMS is sending mail via standard PHP methods so that I can isolate the setup - if you have other suggestions on how to identify the path from CMS to PHP to MX, I would appreciate it. If there are alternate ways to configure emails from the CMS, I'm open to those suggestions as well.

Thank you,