Sprout Forms or Craft not sending email to Craft user email addresses Is there anything in Sprout Forms or Craft that prevents an email being sent to any registered Craft user email address but not other email addresses not in the Craft user list? 

I have a really simple one field form that sends a notification to two email addresses: the one in the submitted form field, as well as the one in the CP Sprout Forms notifications field. 

A weird thing is happening. If I use an email address that is not belonging to a Craft user (either as the typed in notification address or the {email} one) the email is sent (according the server logs) and received. If I use an email address belonging to a  Craft user the email is not sent from Craft (doesn't even register in the server logs).

(The simple form is trying to distil the problem down - it is occurring on all my forms, and was working before but has stopped, possibly the same time as a server change. The PHP mail() function works to said email addresses. I am using latest versions of Craft and Sprout Forms on PHP 7.)

Edit: I can swap email addresses in and out of the user list so when they belong to a user no email is sent, and when they are removed from the user list, the email is sent. This suggests there is nothing wrong with the email address per se or the route through the system, but purely the fact that it is a Craft user email address.

Edit2: When sending to a craft user email address the following craft.log error is made:
Email error: Could not execute: /bin/mini_sendmail (etc)
This doesn't happen when a non-craft user email is used. So suggests to me Sendmail is not properly set up (but why then do other emails get through fine).
As mentioned in the comments, this is likely an email configuration issue and Brad's comment should be taken to heart:


  both sendmail and PHP's mail are notoriously unreliable and shouldn't be used in production environments


Craft should not be treating user emails differently than non-user emails and Sprout Forms isn't doing anything unique when it sends notification emails, it's just piggybacking on what Craft is doing.

Sending your email through a service like Mailgun (or any similar service) will give you more visibility into what's happening and what you may need to troubleshoot. Sprout Email has a Sent Email Element Type that you can enable to help troubleshoot some email deliverability issues on the Craft side of things.

For two good overviews on troubleshooting email errors in Craft, see:


https://craftcms.com/support/troubleshooting-email-errors - Craft Docs
Troubleshooting and debugging errors in PHP - Straight Up Craft




Another direction to consider when troubleshooting an error like this may be to look closer at the From headers and what emails you are sending to. Is it really just Craft Emails vs non-Craft Emails or could there be other patterns? For example, maybe the emails that aren't being received are all gmail addresses. In some cases, if you are sending to a gmail address you own from a gmail address you own, Google may suppress the email from appearing in your inbox.