How to prevent user registration with email addresses from a specific service provider

Thank you, thank you! You pointed me at the right direction. The function I need to modify is actually function user_validate_mail($mail) since I want to check for an email address and I can use the substr function to chop the string and return a value! I am about to try it out :)

Ok I just saw that but looks like a tedious job!

I actually did something much more efficient!! :)

First, I got this file: http://compuweb.com/url-domain-bl.txt

Then, I wrote a simple function in user.modules. The idea is to compare each line in the the spamlist file against the entered email address and check whether the current line in the file is contained within the user entered email address string.

function user_validate_bogus_email($mail) 
{
	
	// reads an entire file and stores it into an array so each line of the file is 
	// stored into a new element in $spamlist_array. 
	/* 
		$spamlist_array[0] = 'This is line 1';
		$spamlist_array[1] = 'This is line 2';
	*/
	$spamlist_array = file("http://example.com/spamlist.txt");

	// iterates till the end of the file where each element of the array is represented as
	// $line_num and the actual value as $value
	/*
		$spamlist_array[$line_num];
	*/
	foreach ($spamlist_array as $line_num => $value) 
	{
		// we want to check whether the current line ($value) is in $mail
		// we are not simply checking for $value because it has some type of character that makes the strpos()
		// function fail (most probably the new line character)
		$realvalue  = substr($value, 0, strlen($value) - 1);	// string to search in, start_pos, length
		$pos = strpos($mail, $realvalue);

		if ($pos === false) 
        {
             // do something if you want
		} 
		else 
		{
			return t('The email address you have entered is banned.<br />If you believe this is caused due to an error, please contact us.');
		}
	}	
}

Then in user_validate_mail, at the very bottom I added:

  else { 
	return user_validate_bogus_email($mail);
  }

So my entire function looks like this:

function user_validate_mail($mail) {
  if (!$mail) return t('You must enter an e-mail address.');
  if (!valid_email_address($mail)) {
    return t('The e-mail address %mail is not valid.', array('%mail' => $mail));
  }
  else { 
	return user_validate_bogus_email($mail);
  }
}

Ok, I know it's a bad practice to mess around with the Drupal modules but at the moment, I don't know how to turn this into a module or an override (haven't RTFMed/Googled enough (links are more than welcome :D) ).