SharePoint 2013 which has been sending emails with no issues since 2014.
This past October, staff noticed that emails being sent to addresses outside of their domain weren't not being sent. This included emails sent to SharePoint users which have email addresses outside the domain and to email addresses not associated with SharePoint users.
The issue is with all external emails, both as part of workflows and not.
We've tried all the commonly recommended troubleshooting. There is a receive connector configured correctly in Exchange. Outgoing email configuration settings are configured correctly in Central Admin and for specific SP applications.
All of the workflows are 2010 workflows.
There are no known changes to the SharePoint configuration that happened between the last time external emails were sent successfully and when they stopped being sent.