Yes, you can still use our mail backup serivce while the domain's DNS is pointing to other DNS servers. What you need to do is to sign up mail backup service for your domain at our web site; then you just need to go the domain's DNS and add smtp.dnsexit.com as the secondary MX for the domain. Remember the backup mail exchanger should have higher preference number (or priority number) than the primary mail exchanger. To find what are the mail exchangers of a domain, you can use the DOS/Linux nslookup command. For example, to search what are the mail exchanger of google.com:
c:>> nslookup -type=mx google.com
google.com mail exchanger = 20 alt1.aspmx.l.google.com.
google.com mail exchanger = 30 alt2.aspmx.l.google.com.
google.com mail exchanger = 10 aspmx.l.google.com.
google.com mail exchanger = 50 alt4.aspmx.l.google.com.
google.com mail exchanger = 40 alt3.aspmx.l.google.com.
In the above example, aspmx.1.google.com is the primary MX. The rest of the servers are all secondary MX.