John Buckman said...
|A great many ISPs offer their corporate customers "virtual email
|domains", which exist only to offer a pretty email address with the
|customer's company name on it. Thus, a number of seemingly
|different domains might point to the same mail server as their
|primary MX host.
|Thus, if you cache the MX records of DNS lookups of your subscriber
|list, and aggregate according to those MX records, (mail.isp.net, as
|opposed the domain portion of the email address, say isp.net) you'll
|have a higher success rate, and save yourself a considerable amount
|of time in avoiding those DNS lookups on every subscriber for every
|message to the list.
So what happens when a mailhost reroutes mail
based on the domain? For instance, email@example.com
and firstname.lastname@example.org might be different addresses,
where foo.com and bar.com are virtual domains on the