I have a list with a workflow customized using SharePoint Designer. It's on the SP2016 platform.
If I am inside the Application (APP) server, I load the site, and I create a list item, the workflow successfully starts and completes, and sends email notifications.
If I am using a normal user machine or the Web Front End (WFE) server, I load the site, and I create a list item, the workflow successfully starts and completes, but does not send email notifications. In the workflow history, you will see the error "The e-mail message cannot be sent. Make sure the outgoing e-mail settings for the server are configured correctly".
Inside the server, the antivirus does not have blocking rules for emails.
I cannot find anything useful in the SharePoint logs. We're currently requesting for Event Viewer and IIS logs.
Note that subscribing to SharePoint alerts are working whether the SharePoint alert was triggered from inside or outside of the server (Example: I subscribed to all events on a list. Whether I create a list item from inside or outside of the server, I still get a SharePoint alert).
Any ideas on what to look for or how to further troubleshoot?