hmm what if the host is down aram?

actually what I know about this is we unfortunately cannot do it
because some email servers will allow you to connect to it and check
if email account exists or not, but most of email server wont let you
do that

I'll try to give real world scenarios here

http://dnsstuff.com/tools/[EMAIL PROTECTED]
   madeye.us. - 204.157.15.31  [Could not connect: Got an unknown RCPT
TO response: 550-"The recipient cannot be verified.  Please check all
recipients of this
550 message to verify they are valid."
]
## [EMAIL PROTECTED] exists, but not a pop3 account, it's forwarded account
so in such cases you'll be blocking a good number of people whose
using mail forwarding accounts


http://dnsstuff.com/tools/[EMAIL PROTECTED]
   easyhttp.com. - 204.157.15.31  [Successful connect: Got a good
response [250 Accepted]]

This one exists 


now lets check with yahoo!
http://dnsstuff.com/tools/[EMAIL PROTECTED]
###  [EMAIL PROTECTED] DOES NT exists, but the server still
responding with Recipient ok

Trying to connect to all mailservers:

   mx3.mail.yahoo.com. - 64.156.215.18  [Successful connect: Got a
good response [250 recipient  ok]]
   mx3.mail.yahoo.com. - 64.156.215.5  [Successful connect: Got a good
response [250 recipient  ok]]
   mx3.mail.yahoo.com. - 64.156.215.6  [Successful connect: Got a good
response [250 recipient  ok]]
   mx3.mail.yahoo.com. - 4.79.181.12  [Successful connect: Got a good
response [250 recipient  ok]]
   mx2.mail.yahoo.com. - 64.156.215.8  [Successful connect: Got a good
response [250 recipient  ok]]
   mx2.mail.yahoo.com. - 67.28.114.35  [Successful connect: Got a good
response [250 recipient  ok]]
   mx2.mail.yahoo.com. - 67.28.114.36  [Successful connect: Got a good
response [250 recipient  ok]]
   mx2.mail.yahoo.com. - 4.79.181.13  [Successful connect: Got a good
response [250 recipient  ok]]
   mx1.mail.yahoo.com. - 67.28.113.10  [Successful connect: Got a good
response [250 recipient  ok]]
   mx1.mail.yahoo.com. - 67.28.113.11  [Successful connect: Got a good
response [250 recipient  ok]]
   mx1.mail.yahoo.com. - 4.79.181.14  [Successful connect: Got a good
response [250 recipient  ok]]
   mx1.mail.yahoo.com. - 4.79.181.15  [Successful connect: Got a good
response [250 recipient  ok]]
   mx4.mail.yahoo.com. - 68.142.202.11  [Successful connect: Got a
good response [250 recipient  ok]]
   mx4.mail.yahoo.com. - 68.142.202.12  [Successful connect: Got a
good response [250 recipient  ok]]
   mx4.mail.yahoo.com. - 66.218.86.156  [Successful connect: Got a
good response [250 recipient  ok]]
   mx4.mail.yahoo.com. - 67.28.113.19  [Successful connect: Got a good
response [250 recipient  ok]]


What I'm thinking of is the best way to validate an account is to 
1-) have a black list of known spam domains
2-) verify accounts by email activation link and/or code

that was a tip to answer How to check if the email exists!

now if you want to check the Domain exists or not maybe doing a
ping/pong like with whois server

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