Laravel queued mail not received

I'm trying to use queue for my email sending on Laravel 5.4

Mail::to('email@example.com')->queue(new NotifyCustomer($client));

What works: QUEUE_DRIVER=sqs or database with Mailgun

What works: QUEUE_DRIVER=sync with Mailgun with domain up******.com

What doesn't work: QUEUE_DRIVER=sqs or database with domain up******.com. But, it works for other domains.

Is this an issue with domain up******.com MX entry or DNS setup or something not working at Illuminate\Mail\SendQueuedMailable?

Thanks in advance.

via Chebli Mohamed