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A Word about Email Validation
Email validation is a tricky process. aspNetMX has made this process
incredibly simple and powerful. The only correct way to verify an email
address exists is to send a mailbox an email and see if you receive a
bounce back email or NDR (Non-Deliverable Receipt). aspNetMX goes as far as
possible in attempting to validate email addresses WITHOUT sending an
actual email.

There are a couple of issues to be aware of when validating email addresses
at the MXValidateLevel.Mailbox level.

Positive Validation when a Mailbox doesn't exist
Some mail servers always return a positive response that a mailbox exists,
and it is only until an email is sent to a mailbox, that the server
responds with a negative response saying a mailbox does not exist.
Microsoft's exchange server is notorious for doing this. However, we have
optimized aspNetMX to help with this situation, and return faster results
against these known servers.

MXValidateLevel.Mailbox can be a Time Intensive Process
To better explain what happens under the covers when an email addresses is
tested to Mailbox level, the following steps occur:

   1. The email addresses is syntactically checked.
   2. If the email address is valid, a DNS Lookup for MX Records is made.
This involves network calls to DNS Servers to see if MX Records exist. If
the binary MX Records exist, and are returned to aspNetMX, aspNetMX turns
the records into a usable form.
   3. If the MX Records return the names of the Mail Servers, and not the
IP addresses of the mail servers, another DNS call is made to lookup the
actual IP addresses.
   4. Once the IP addresses have been determined, another network call is
made, this time to the email address's SMTP server. If the mail server is
not responding, and there are fail over MX Records, aspNetMX will resolve
each of the additional MX Records to IP addresses and then attempt to make
a successful SMTP connection.
   5. Once a SMTP session has been established, standard SMTP commands are
issued against the mail sever to determine if a mailbox exists for the
email address. To protect against email harvesting, some mail servers will
always return a positive response, saying a mailbox exists, when in fact,
it doesn't. Also, a DNS Server or SMTP server may be down at the very
instance you are attempting to validate an email address. Thus aspNetMX
could accidentally mark that email address as not valid, when in fact it
is. It's recommended that you test failed email addresses a couple of
times, at different times, to verify they are in fact bad.

aspNetMX has been highly optimized, unfortunately, most of the
time-intensive issues that affect email validation are outside of the
control of aspNetMX. These include the across-the-internet or
across-the-network DNS lookups, and the across-the-internet SMTP calls.
Depending upon your network speed, and the remote SMTP server, the whole
process may be completed in a few milliseconds, however, we've seen it take
as long as 60 seconds. To help with this, aspNetMX utilizes a number of
built-in higher-performance techniques. Check out aspNetMX's High
Performance Features.

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Andrew Faust
andrew[at]andrewfaust.com
http://www.andrewfaust.com