In other discussions, I was made aware that Stalwart limits one Mailbox per alias. Other uses, according to the discussion, should use mailing lists instead.
I still believe users would be better served with the possibility of haivng more than one mailbox per alias if they choose. This is specially useful for oganizations which have a public e-mail but which can be answered from different people.
This is different than having mailing lists, even though the usage is similar.
You may want to look at Groups if you want to allow sending from a shared email address. This doesn’t have quite the same ergonomics as common aliases as it uses a shared mailbox instead, but it’s a good fit for this scenario I think.