I have configured 2 mail servers (primary and secondary) and is running concurrently. Mydomain.com is now pointing to my server A (primary). When server A is down, I use 'dns thingy' to change domain NS records to point to server B (secondary), so all mails can reach server B and all users can send/receive from server B without interruption. When server A is up again, I switch back to server A, but users are now unable to retrive mails that are stuck in server B.
How can users retrive their emails from server B when I switch back to server A? Do I configure MX records or is there a way to migrate mails from server B to A? FYI, I'm using Fedora Core 4.