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As part of a web application I am building, I must check if a particular email address exists. This check may be performed at around 1000 times an hour at a given server. Is there any mail server security policy that would prevent this?

Edit: I do not mean any particular server. Would there be any reason for throttling to be enabled anywhere.

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What reason do you have to do this? If you use an opt-in mailing list, then you ought to be able to verify individual e-mails on sign-up. Are you getting 1000 sign-ups an hour? –  Lèse majesté Nov 23 '10 at 15:55
    
Not me. I am the developer of the application. The users of the application could get that many opt-ins an hour. Furthermore, being able to verify the email address even before sending an email seemed to be a good way to prevent spurious subscriptions. So I am exploring if this is something I should be implementing. –  rsman Nov 23 '10 at 16:18
    
Ah, yea, as John Conde explained, spammers pretty much ruined this one for us. –  Lèse majesté Nov 24 '10 at 16:35

2 Answers 2

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Not only can you expect it to be throttled, but most likely that functionality will be turned off. This is because spammers use this to confirm email addresses.

If I were you I wouldn't use this technique as it sounds suspicious to me. You may find your site blacklisted as a spammer which definitely is counterproductive to your goals. Simply send a confirmation email to your users with a confirmation link inside. Until their email is confirmed limit their account or prevent access altogether. This functionality is commonly used and easy to implement.

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The only way to be sure that an email address exists is to send an email to it and check that they opened it. The user might have very strict spam filtering that filters your email away. The email address still exists and yet you can't send anything to it.

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