Can't Receive Emails On My Set-Up Email Accounts (550 5.1.1 Requested action not taken: mailbox unavailable)

Hello guys!

I really need your help, as I am on the verge of frustration right now.

For days, I have been trying all I can to set up my email services. I host my domain on AWS. After creating users on Virtualmin, I installed Roundcube. I could send messages, even though it shows I am not authorized... But the bigger problem is that I can't receive messages. Every time I try sending from my gmail account, I keep getting this error:

550 5.1.1 Requested action not taken: mailbox unavailable

Please, help me. I am tired.