Hello esmi - thanks for the response!
I've done steps 1-3 on our staging server without any luck. I'm a little wary about trying step 4 since we are using multisite - but I'll give it a shot and report back.
As a temporary fix I've been setting up SMTP on all of our sites that have registration forms (we have close to 50 sites on the network) - but this is problematic as passwords on email accounts in our organization expire every 3 months. On the plus side, the plugin I used to do this lets you toggle between SMTP and the default WP phpmailer. When I run a test email using the WP phpmailer, I get this: (redacted has been substituted where sensitive information appeared)
[Log moderated. Please use the pastebin]
The SMTP debugging output is shown below:
Could not instantiate mail function.