DomainTools has opened up a huge database that matches domain names to the mail servers they use.
A search on ReverseMX.com for a domain name returns the mail servers that domain uses. In reverse, you can search for a mail server or IP address and find out which domains use it.
For example, a query for one of Google’s mail servers will spit back a short list of some of the domains that use Google for their email, along with an aggregate domain count.
DomainTools said in a press release:
ReverseMX can be used by a wide audience – basically anyone interested in researching the footprint of small or large email providers. For example, users can analyze which mail servers’ domains are using certain email providers, or how Microsoft’s hosted email is doing against Gmail or Yahoo.
The data currently covers the 130 million domains registered under .com, .edu, .net, .org, .info, .biz, and .us – the largest TLDs for which zone files are freely available.
DomainTools has already uncovered a few interesting factoids, such as that 30 million domain names use Go Daddy for their email, making it easily the largest provider.
The service also interrogates domains’ SPF records to work out which IP addresses are authorized to send email for any given domain.
I can imagine ReverseMX being useful for researchers in the security industry (and their spammer adversaries?).
But unlike DomainTools’ other services, it does not immediately appear to be something that many people in the domain name industry will find themselves using on a daily basis.