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Up until now, if you had an alias, virtual alias or distribution list with multiple targets, and one of those targets was a FastMail address that didn't exist as an alias/username, then that address would be silently ignored.
For instance, say you have a domain example.com with an alias "firstname.lastname@example.org" that targets "email@example.com , firstname.lastname@example.org". And say you have an alias for "email@example.com", but no alias for "firstname.lastname@example.org". Previously this would have worked without any error, with email sent to email@example.com going to only firstname.lastname@example.org, and the email@example.com target being ignored because it wasn't a valid address.
Now however, sending an email to firstname.lastname@example.org will deliver the email to email@example.com, but will also generate a bounce message back to the sender, explaining that the email couldn't be delivered to firstname.lastname@example.org, one of the targets of the alias.