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 "sales@example.com" that targets "sam@example.com , joe@example.com". And say you have an alias for "sam@example.com", but no alias for "joe@example.com". Previously this would have worked without any error, with email sent to sales@example.com going to only sam@example.com, and the joe@example.com target being ignored because it wasn't a valid address.

Now however, sending an email to sales@example.com will deliver the email to sam@example.com, but will also generate a bounce message back to the sender, explaining that the email couldn't be delivered to joe@example.com, one of the targets of the alias.