It's not exactly clear how you're doing backups, perhaps copying messages via IMAP, but generally you can specify your MX's records priority, here's an example with Google :
Quote:
dig @ns1.google.com gmail.com mx
; <<>> DiG 9.3.4-P1 <<>> @ns1.google.com gmail.com mx
; (1 server found)
;; global options: printcmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 6531
;; flags: qr aa rd; QUERY: 1, ANSWER: 5, AUTHORITY: 4, ADDITIONAL: 6
;; QUESTION SECTION:
;gmail.com. IN MX
;; ANSWER SECTION:
gmail.com. 3600 IN MX 50 gsmtp147.google.com.
gmail.com. 3600 IN MX 50 gsmtp183.google.com.
gmail.com. 3600 IN MX 5 gmail-smtp-in.l.google.com.
gmail.com. 3600 IN MX 10 alt1.gmail-smtp-in.l.google.com.
gmail.com. 3600 IN MX 10 alt2.gmail-smtp-in.l.google.com.
Which means : the name server ns1.google.com knows that the domain 'gmail.com' has few MX records, and the highest priority is email server with name "gmail-smtp-in.l.google.com". If it fails, for some reason, the next upper number server will take care of the mail delivery.
Actually, if you ping each of the top 3 server, you'll notice that they are in a totally different network segments, which is a good practice. Imaging an erroneous router rule, for denying access to 10.101.1.0/24
Just an example