Re: Finding if email addresses exist?

Edit: I stand corrected. Don't do this, see explanation below. Thanks to all of the wise and cordial monks who have shown me the light on this topic, I could have very easily been flamed for making an unwise suggestion. :)

As jeyroz said, Email::Valid is a good tool for a quick and dirty check. However, there's one more thing that you can do. Here's a quick snippet from a conversation with an e-mail server:

[tex@smtp-client ~]$ telnet smtp-server.example.com 25
Trying xxx.xx.xxx.x...
Connected to smtp-server.example.com
Escape character is '^]'.

MAIL FROM: <user@smtp-client.example.com>
250 Ok
RCPT TO: <valid-user@smtp-server.example.com>
250 Ok
[download]

So, at this point you can break the connection knowing you have a valid user. Here's the same conversation, slightly modified:

[tex@smtp-client ~]$ telnet smtp-server.example.com 25
Trying xxx.xx.xxx.x...
Connected to smtp-server.example.com
Escape character is '^]'.

MAIL FROM: <user@smtp-client.example.com>
250 Ok
RCPT TO: <invalid@bioinformatics.rit.edu>
450 <invalid@bioinformatics.rit.edu>: Recipient address rejected: User
+ unknown in local recipient table
[download]

So, if you write a program that speaks SMTP well, you can check a mail server for valid users without actually sending an e-mail. There's no guarantee that the server won't bounce the message after accepting it, but this just shows another trick beyond just using Email::Valid.

Comment on Re: Finding if email addresses exist? Select or Download Code

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Re^2: Finding if email addresses exist? by merlyn (Sage) on Jun 21, 2005 at 01:11 UTC
And the FAQ also says that's unreliable. There are servers that say "sure, go ahead" for every address tested that way. And there are servers that say "no, we won't tell you, try sending first" for every email tested that way. Hence, you'll get both false positives and false negatives for such an approach, and far greater than your true reality hits. Hence, the FAQ's wisdom should be followed: you can't get there from here. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re^2: Finding if email addresses exist? by devnul (Monk) on Jun 21, 2005 at 01:51 UTC
This is important enough, I feel compelled to re-iterate what Merlyn just said. Please do not validate email addresses by looking for mail server replies like this. The only thing you can rely upon by doing this is that what you are getting back is 100% unreliable. - dEvNuL	[reply]
Re^2: Finding if email addresses exist? by mikeraz (Friar) on Jun 21, 2005 at 03:25 UTC
Among the other reasons to not use this approach mentioned already is . . . Some spam fighting systems, mine included, monitor the maillog and quickly throw up a firewall rule blocking access to the system as soon as delivery is attempted to an invalid address. The object is to block dictionary attacks. The benefit is that your system no longer processes email destined for bogus addresses. There's a side effect of the sender being cut off after the first invalid address so any spam addressed to multiple adressees will never get delivered to legit addressees either. Be Appropriate && Follow Your Curiosity	[reply]
Re^3: Finding if email addresses exist? by ww (Archbishop) on Jun 21, 2005 at 15:27 UTC
...but a horrid problem for the lex-dysic... whose wife has personal and business accounts: fi_lname@work... and lname_fi@personal... So if I botch an address with lname_fi@work..., her server there is gonna, forever afterward, bounce (or worse, simply dev\nul it) until she cleans my IP out of the filter? Agreed (I'D YELL AGREEMENT! but that would be impolite... but isn't that solution rather draconian (or, alternately, is there something here I don't understand)?	[reply]
Re^4: Finding if email addresses exist? by mikeraz (Friar) on Jun 23, 2005 at 03:50 UTC
Dyslexia? That's what address books are for. As it is I lock 'em out for six minutes. A spammer goes on. A legit email will eventually bounce and then can send again. Draconian? My tiny little home system went from processing over 30,000 incoming emails a day to ~4,800. Be Appropriate && Follow Your Curiosity	[reply]