Troubleshooting: SMTP Error (550): Failed to add recipient (5.1.1 address rejected: User unknown in virtual alias table)

Hi this problem has bogged me down, and I noticed there were few these 550 5.1.1 errors already in forum. Unfortunately haven't found the solution.

This has do with one email address, which uses a 3rd party email solution provider: user@example.com, that gives me the error when I try to send from user@ejemplo.com. error message: SMTP Error (550): Failed to add recipient "user@example.com" (5.1.1 user@example.com: Recipient address rejected: User unknown in virtual alias table).

Background (this is probably important): Both example.com and ejemplo.com are hosted on my virtualmin, they were recently migrated here, so I wonder if there is a local conflict? In short:

• Both example.com and ejemplo.com are hosted on my domain
• example.com uses a 3rd party email service
• ejemplo.com uses roundcube on virtualmin
• sending a message from user@ejemplo.com to example.com gives the error message above.
• sending messages to other emails (gmail etc) works fine from user@ejemplo.com
• when user@ejemplo.com was not hosted on virtualmin, it sent messages to user@example

Thanks for any insights! Definitely contacting my email provider for example.com, but I want to understand the underlying issue here.

thanks so much.