Lookup using MX record to validate mail serverTag(s): Networking
The best you can do to verify if an email address is real is to verify if there is a mail server registered to the domain name.
import java.util.Hashtable;
import javax.naming.*;
import javax.naming.directory.*;
public class MXLookup {
public static void main( String args[] ) {
if( args.length == 0 ) {
System.err.println( "Usage: MXLookup host [...]" );
System.exit( 99 );
}
for( int i = 0; i < args.length; i++ ) {
try {
System.out.println( args[i] + " has " +
doLookup( args[i] ) + " mail servers" );
}
catch( Exception e ) {
System.out.println(args[i] + " : " + e.getMessage());
}
}
}
static int doLookup( String hostName ) throws NamingException {
Hashtable env = new Hashtable();
env.put("java.naming.factory.initial",
"com.sun.jndi.dns.DnsContextFactory");
DirContext ictx = new InitialDirContext( env );
Attributes attrs =
ictx.getAttributes( hostName, new String[] { "MX" });
Attribute attr = attrs.get( "MX" );
if( attr == null ) return( 0 );
return( attr.size() );
}
}>java MXLookup rgagnon.com realhowto.com rgagnon.com has 1 mail servers realhowto.com : DNS name not found [response code 3]
From T. Orbaker, more infos on that subject (thanks to him).
There are other methods by which to validate an email address to a higher degree of than just the mail server.
One:
Use the VRFY command (see RFCs 821/2821). Because this was
abused by spammers, it have typically been disabled on most
mail servers. Some recent servers don't even support this
command as they are so frequently shut off.
When it works, connect to the server, issue the HELO command
and then send 'VRFY Two (better method): Once you have connected, you create the SMTP envelope, but you
don't put anything in it. This is the point at which most
servers will give up the dirt on whether or not an address is
valid. If an envelope cannot be built, we know that the
address is invalid.
The reason for connecting in order of preference:
Imagine ABC company has an Internet conneciton and runs their
own mail server for abc.com. To prevent bounces and other mail
errors if their connection or server should be down, their
provider isp.com agrees to set up a 'store and forward' scheme
for their mail. If abc.com is not available, then isp.com gets
the message and when abc.com is again available, the message
gets forwarded. The MX records would look something like:
If these are not checked in the proper order, there will be
no errors for invalid addresses.
Yahoo appears to use a store and forward mechanism to its own
internal servers, thus conclusively verifying a yahoo address
is not possible. I suspect that hotmail is the same.
It is not possible to verify an address on a domain that uses
a catch-all account as the catch account will receive the mail
(it does, however, mean that someone will at least SEE the
message).
S.Boerner has this comments about this HowTo :
The method SMTP::getMX() in the second example throws an
ArrayIndexOutOfBoundsException while getting the mailhost via
the "A" attribute at:
A list a possible SMTP Reply Codes:
Connect to the servers determined by your code snippet. BUT
(and the code below doesn't do this) they must be tried from
lowest preference to highest to be absolutely correct.
MX 1 abc.com
MX 5 isp.com
import java.io.*;
import java.net.*;
import java.util.*;
import javax.naming.*;
import javax.naming.directory.*;
public class SMTP {
private static int hear( BufferedReader in ) throws IOException {
String line = null;
int res = 0;
while ( (line = in.readLine()) != null ) {
String pfx = line.substring( 0, 3 );
try {
res = Integer.parseInt( pfx );
}
catch (Exception ex) {
res = -1;
}
if ( line.charAt( 3 ) != '-' ) break;
}
return res;
}
private static void say( BufferedWriter wr, String text )
throws IOException {
wr.write( text + "\r\n" );
wr.flush();
return;
}
private static ArrayList getMX( String hostName )
throws NamingException {
// Perform a DNS lookup for MX records in the domain
Hashtable env = new Hashtable();
env.put("java.naming.factory.initial",
"com.sun.jndi.dns.DnsContextFactory");
DirContext ictx = new InitialDirContext( env );
Attributes attrs = ictx.getAttributes
( hostName, new String[] { "MX" });
Attribute attr = attrs.get( "MX" );
// if we don't have an MX record, try the machine itself
if (( attr == null ) || ( attr.size() == 0 )) {
attrs = ictx.getAttributes( hostName, new String[] { "A" });
attr = attrs.get( "A" );
if( attr == null )
throw new NamingException
( "No match for name '" + hostName + "'" );
}
// Huzzah! we have machines to try. Return them as an array list
// NOTE: We SHOULD take the preference into account to be absolutely
// correct. This is left as an exercise for anyone who cares.
ArrayList res = new ArrayList();
NamingEnumeration en = attr.getAll();
while ( en.hasMore() ) {
String x = (String) en.next();
String f[] = x.split( " " );
if ( f[1].endsWith( "." ) )
f[1] = f[1].substring( 0, (f[1].length() - 1));
res.add( f[1] );
}
return res;
}
public static boolean isAddressValid( String address ) {
// Find the separator for the domain name
int pos = address.indexOf( '@' );
// If the address does not contain an '@', it's not valid
if ( pos == -1 ) return false;
// Isolate the domain/machine name and get a list of mail exchangers
String domain = address.substring( ++pos );
ArrayList mxList = null;
try {
mxList = getMX( domain );
}
catch (NamingException ex) {
return false;
}
// Just because we can send mail to the domain, doesn't mean that the
// address is valid, but if we can't, it's a sure sign that it isn't
if ( mxList.size() == 0 ) return false;
// Now, do the SMTP validation, try each mail exchanger until we get
// a positive acceptance. It *MAY* be possible for one MX to allow
// a message [store and forwarder for example] and another [like
// the actual mail server] to reject it. This is why we REALLY ought
// to take the preference into account.
for ( int mx = 0 ; mx < mxList.size() ; mx++ ) {
boolean valid = false;
try {
int res;
Socket skt = new Socket( (String) mxList.get( mx ), 25 );
BufferedReader rdr = new BufferedReader
( new InputStreamReader( skt.getInputStream() ) );
BufferedWriter wtr = new BufferedWriter
( new OutputStreamWriter( skt.getOutputStream() ) );
res = hear( rdr );
if ( res != 220 ) throw new Exception( "Invalid header" );
say( wtr, "EHLO orbaker.com" );
res = hear( rdr );
if ( res != 250 ) throw new Exception( "Not ESMTP" );
// validate the sender address
say( wtr, "MAIL FROM: <tim@orbaker.com>" );
res = hear( rdr );
if ( res != 250 ) throw new Exception( "Sender rejected" );
say( wtr, "RCPT TO: <" + address + ">" );
res = hear( rdr );
// be polite
say( wtr, "RSET" ); hear( rdr );
say( wtr, "QUIT" ); hear( rdr );
if ( res != 250 )
throw new Exception( "Address is not valid!" );
valid = true;
rdr.close();
wtr.close();
skt.close();
}
catch (Exception ex) {
// Do nothing but try next host
}
finally {
if ( valid ) return true;
}
}
return false;
}
public static void main( String args[] ) {
String testData[] = {
"tim@orbaker.com", // Valid address
"fail.me@nowhere.spam", // Invalid domain name
"arkham@bigmeanogre.net", // Invalid address
"nosuchaddress@yahoo.com" // Failure of this method
};
for ( int ctr = 0 ; ctr < testData.length ; ctr++ ) {
System.out.println( testData[ ctr ] + " is valid? " +
isAddressValid( testData[ ctr ] ) );
}
return;
}
}
while ( en.hasMore() ) {
String x = (String) en.next();
String f[] = x.split( " " );
if ( f[1].endsWith( "." ) )
The "A" attribute returns only an address list, so f.length is always 1.
I used something like:
boolean hasMX = "MX".equals(attr.getID());
and later
if (hasMX)
{
mailhost = f[1];
}
else
{
mailhost = f[0];
}
Here my proposed fix :
import java.io.*;
import java.net.*;
import java.util.*;
import javax.naming.*;
import javax.naming.directory.*;
public class SMTPMXLookup {
private static int hear( BufferedReader in ) throws IOException {
String line = null;
int res = 0;
while ( (line = in.readLine()) != null ) {
String pfx = line.substring( 0, 3 );
try {
res = Integer.parseInt( pfx );
}
catch (Exception ex) {
res = -1;
}
if ( line.charAt( 3 ) != '-' ) break;
}
return res;
}
private static void say( BufferedWriter wr, String text )
throws IOException {
wr.write( text + "\r\n" );
wr.flush();
return;
}
private static ArrayList getMX( String hostName )
throws NamingException {
// Perform a DNS lookup for MX records in the domain
Hashtable env = new Hashtable();
env.put("java.naming.factory.initial",
"com.sun.jndi.dns.DnsContextFactory");
DirContext ictx = new InitialDirContext( env );
Attributes attrs = ictx.getAttributes
( hostName, new String[] { "MX" });
Attribute attr = attrs.get( "MX" );
// if we don't have an MX record, try the machine itself
if (( attr == null ) || ( attr.size() == 0 )) {
attrs = ictx.getAttributes( hostName, new String[] { "A" });
attr = attrs.get( "A" );
if( attr == null )
throw new NamingException
( "No match for name '" + hostName + "'" );
}
// Huzzah! we have machines to try. Return them as an array list
// NOTE: We SHOULD take the preference into account to be absolutely
// correct. This is left as an exercise for anyone who cares.
ArrayList res = new ArrayList();
NamingEnumeration en = attr.getAll();
while ( en.hasMore() ) {
String mailhost;
String x = (String) en.next();
String f[] = x.split( " " );
// THE fix *************
if (f.length == 1)
mailhost = f[0];
else if ( f[1].endsWith( "." ) )
mailhost = f[1].substring( 0, (f[1].length() - 1));
else
mailhost = f[1];
// THE fix *************
res.add( mailhost );
}
return res;
}
public static boolean isAddressValid( String address ) {
// Find the separator for the domain name
int pos = address.indexOf( '@' );
// If the address does not contain an '@', it's not valid
if ( pos == -1 ) return false;
// Isolate the domain/machine name and get a list of mail exchangers
String domain = address.substring( ++pos );
ArrayList mxList = null;
try {
mxList = getMX( domain );
}
catch (NamingException ex) {
return false;
}
// Just because we can send mail to the domain, doesn't mean that the
// address is valid, but if we can't, it's a sure sign that it isn't
if ( mxList.size() == 0 ) return false;
// Now, do the SMTP validation, try each mail exchanger until we get
// a positive acceptance. It *MAY* be possible for one MX to allow
// a message [store and forwarder for example] and another [like
// the actual mail server] to reject it. This is why we REALLY ought
// to take the preference into account.
for ( int mx = 0 ; mx < mxList.size() ; mx++ ) {
boolean valid = false;
try {
int res;
//
Socket skt = new Socket( (String) mxList.get( mx ), 25 );
BufferedReader rdr = new BufferedReader
( new InputStreamReader( skt.getInputStream() ) );
BufferedWriter wtr = new BufferedWriter
( new OutputStreamWriter( skt.getOutputStream() ) );
res = hear( rdr );
if ( res != 220 ) throw new Exception( "Invalid header" );
say( wtr, "EHLO rgagnon.com" );
res = hear( rdr );
if ( res != 250 ) throw new Exception( "Not ESMTP" );
// validate the sender address
say( wtr, "MAIL FROM: <tim@orbaker.com>" );
res = hear( rdr );
if ( res != 250 ) throw new Exception( "Sender rejected" );
say( wtr, "RCPT TO: <" + address + ">" );
res = hear( rdr );
// be polite
say( wtr, "RSET" ); hear( rdr );
say( wtr, "QUIT" ); hear( rdr );
if ( res != 250 )
throw new Exception( "Address is not valid!" );
valid = true;
rdr.close();
wtr.close();
skt.close();
}
catch (Exception ex) {
// Do nothing but try next host
ex.printStackTrace();
}
finally {
if ( valid ) return true;
}
}
return false;
}
public static void main( String args[] ) {
String testData[] = {
"real@rgagnon.com",
"you@acquisto.net",
"fail.me@nowhere.spam", // Invalid domain name
"arkham@bigmeanogre.net", // Invalid address
"nosuchaddress@yahoo.com" // Failure of this method
};
for ( int ctr = 0 ; ctr < testData.length ; ctr++ ) {
System.out.println( testData[ ctr ] + " is valid? " +
isAddressValid( testData[ ctr ] ) );
}
return;
}
}
A note sent by M. Donders.
While using the code I noticed a problem with greylisting.
Greylisting, is an anti spam attempt which denies service
for the first time the triple of client, sender and address
reaches the smtp server. Greylisting will send back a
temporary error (450) and therefore the address will be
denied. In this case it probably is better to accept the
address as verified, because there is no better information
available at that moment.
Code Description
211 System status, or system help reply.
214 Help message.
220 Domain service ready.
Ready to start TLS.
221 Domain service closing transmission channel.
250 OK, queuing for node node started.
Requested mail action okay, completed.
251 OK, no messages waiting for node node.
User not local, will forward to forwardpath.
252 OK, pending messages for node node started.
Cannot VRFY user (e.g., info is not local),
but will take message for this user and attempt delivery.
253 OK, messages pending messages for node node started.
354 Start mail input; end with
RFC0821