Scenario:
I have a contact form on my web app, it gets alot of spam.
I am validating the format of email addresses loosely i.e. ^.+@.+\..+$
I am using a spam filtering service (defensio) but the spam scores returned are overlapping with valid messages. At a threshold of 0.4 some spam gets through and some customer's questions are wrongly thrown in a log and an error displayed.
All of the spam messages use fake email addresses e.g. zxmzxm@ywduasm.com
Dedicated PHP5 Linux server in US, mysql, logging spam only, emailing the non spam messages (not stored).
Proposal:
Use php's checkdnsrr(preg_replace(/^.+?@/, '', $_POST['email']), 'MX') to check the email domain resolves to a valid address, log to file, then redirect with an error for messages that don't resolve, proceed to the spam filter service as before for addresses that do resolve according to checkdnsrr().
I have read (and i am sceptical about this myself) that you should never leave this type of validation up to remote lookups, but why?
Aside from connectivity issues, where i will have bigger problems than a contact form anyway, is checkdnsrr going to encounter false positives/negatives?
Would there be some address types that wont resolve? gov addresses? ip email addresses?
Do i need to escape the hostname i pass to checkdnsrr()?
Solution: A combination of all three answers (wish i could accept more than one as a compound answer).
I am using:
$email_domain = preg_replace('/^.+?@/', '', $email).'.';
if(!checkdnsrr($email_domain, 'MX') && !checkdnsrr($email_domain, 'A')){
//validation error
}
All spam is being logged and rotated. With a view to upgrading to a job queue at a later date.
Some comments were made about asking the mail server for the user to verify, i felt this would be too much tra