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: email@example.com, that gives me the error when I try to send from firstname.lastname@example.org. error message: SMTP Error (550): Failed to add recipient "email@example.com" (5.1.1 firstname.lastname@example.org: 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:
Thanks for any insights! Definitely contacting my email provider for example.com, but I want to understand the underlying issue here.
thanks so much.