Email Send Troubleshooting

I am trying to get this plugin to work; it looks great, but am struggling. I am using mailgun as my smtp provider. I can get sent emails to show up at one address, but not other test email addresses at Google, Comcast, AOL and two addresses hosted at Godaddy (one works, the other doesn’t). All of these addresses work if I send an email manually. I’ve also tried smpt settings for other providers besides mailgun. I’m doing this all on a test site at AWS Lightsail. Hoping for help on troubleshooting this. The plugin indicates that the sends work, but I don’t get them in the various inboxes except for one. Thank you.