I have been testing SMTP and its related DNS register, but I already tested it and what I have achieved is be able to send emails from the same email account (email@example.com) from different servers, configuring the domain's DNSs like this:
mydomain.com A | mail | points to *vps 1 ip addr* A | mail2 | points to *vps 2 ip addr* A | pop | points to *vps 1 ip addr* MX | mydomain.com | mail handled by mail.mydomain.com (priority 10) MX | mydomain.com | mail handled by mail2.mydomain.com (priority 20) TXT | _domainkey | t=y; o=~; TXT | mydomain.com | v=spf1 a mx ip4: *vps 1 ip addr* ?all TXT | mydomain.com | v=spf1 a mx ip4: *vps 2 ip addr* ?all
I can even log in both account from clients like thunderbird and apparently everything works fine, but only one server can receive mails (it's obvious I guess, and I think it depends of priority set in MX records, couldn't test it yet).
So I stopped exim service from vps1 and tried to send a email expecting that would be sent to vps2 since vps1 wouldn't respond, but the sender never tried to send it to vps2...
Is possible to configure it in any way vps2 would receive mails just in case vps1 doesn't response?
I'm using exim under VestaCP in both servers
PD: I just realized that I've not set any register like pop pointing to my vps2