We're planning a Zimbra deployment for a client and they made an unusual request: being able to route outgoing mail differently according to the "From" address used.
Let me explain.
This company have two geographically assigned domains: domain.qc.ca and domain.on.ca; and mail sent to domain.qc.ca is received through a mail gateway located in Quebec while mail sent to domain.on.ca passes through a mail gateway located in Ontario. These SMTP gateways are not Zimbra boxes. This is done with the magic of DNS MX records.
Once the messages are inside our client's network, they are all routed to the Zimbra server. Also, mail sent to
user@domain.qc.ca gets to the same mailbox as a message sent to
user@domain.on.ca. So far so good.
When users send emails to the external world, all outgoing messages go through the Quebec SMTP gateway, whether they're using the domain.qc.ca or domain.on.ca "From" or "Reply-To" address.
Our client wants to have mail sent using domain.qc.ca to get through the Quebec gateway and reach the Internet from there while mail using domain.on.ca be routed through the Ontario SMTP gateway and reach the Internet from an Ontario based IP address.
Is it possible to build this kind of setup with Zimbra? (We're willing to work at a lower level and tweak Postfix if necessary.)
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Multiple domains with multiple SMTP points of presence 
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