I have 2 mailservers (primary - site1) and (secondary - site2) in two separated locations (sites).

If my primary mailserver is unavailable, then the secondary takes place and start receiving email messages.

How my internal clients in the primary location (site1) will send the messages to secondary server?An email coming from outside it's OK, because we have 2 MX entries (primary and secondary mailservers), but the internal client (same network) dont send the email messages to MX server, they deliver the message directly to mailserver.

Many Tks!