Hi. I would do the following:
1 ) add the alert to list of Triggered Alerts so you can truly see the alert is triggering.
When Triggered.. Add to List of Triggered Actions
2) Search the search head to make sure the email is being sent
I did a test with a bad email and I did not get an error back from our MTA, unfortunately. But at least you can see the email attempting to be sent. I saw something like this
2018-08-20 23:12:02,263 +0000 INFO sendemail:134 - Sending email. subject="Splunk Alert: Test message", results_link="http://myserver.xx.com:1234/splunkunk/app/launcher/@go?sid=scheduler__burwell__launcher__RMD5dd4d34f24a9c4022_at_1534806720_6612", recipients="[firstname.lastname@example.org']", server="mymailserver.com:25"