A Bit More Difficult
Setup DNS: xyz.com. IN MX 10 mailrelay.xyz.com. bite.com. IN MX 10 mailrelay.xyz.com. me.com. IN MX 10 mailrelay.xyz.com.
Mailrelay.m4: MASQUERADE_AS(xyz.com) FEATURE(mailertable) FEATURE(masquerade_entire_domain)
mailertable: xyz.com relay:[mailserver.xyz.com] bite.com relay:[mail.bite.com] me.com relay:[mail.me.com]