Some more information is probably required here, because in general, this works. Do both the outgoing mail and the reply have the ticket ID in the subject? Post the source for both mails if you can, that would probably make it easier for us to determine what’s going on here.
No it doesn’t! Maybe I didn’t explain clearly enough.
I, as an agent, can open a new ticket on behalf of a client by sending an email to the customer CCing Zammad email address. This automatically creates the ticket associated to the customer’s company. So the first email doesn’t have a ticket id, yet. I expect Zammad to link the two mails by References header, as there’s no other way to do that