I have 2 suggestions for consideration:
Show a notification that an email failed to send in the UI. Include enough information to troubleshoot (or instructions on how to get that information). Currently I see no notifications.
If this command fails, give more information in the console:
sudo apps/phabricator/htdocs/bin/mail send-test
Right now I get is this message and then a stack trace:
Reading message body from stdin...
[2018-08-28 16:06:29] EXCEPTION: (SimpleEmailServiceException) Unexpected HTTP status while making request to Amazon SES: expected 200, got 400.