In our sandboxes Developer orgs, We have recently noticed as of May 30 2019 that emails ( workflow, process builder, apex ) have stop being sent when the invoking user's email ends in '.invalid'. Understand Salesforce changes the users email to prevent sending emails in Sandboxes on accident but in the past logging in as that user and performing workflow / application actions still sent email alerts.
We have a case open with Salesforce.. I am curious if anyone else is seeing this?
Have documented emails that were sent via workflow email alerts prior to May 31st so this is not past behavior.
note the recent post here where this first mentioned: https://success.salesforce.com/answers?id=9063A000000lXhVQAU
Took us a bit to confirm this so hopeful this helps the next person..