以编程方式识别和验证邮件服务器

Question

Like did in the mailtester website , I have to identify & validate-the-existance-of mail server, programmatically.

I've checked the web deeply but I don't see any tutorial, how-to-do guide, api, library, or online course.

I will not use the program for any spam purpose like done in the mentioned website (no declared privacy issues about anti-spam, or no declaration for illegal usage & storage of valid email address) . By the way, mail server identification is small part of the project I'm now working on, but I have to implement it.

Please, answer any of the, but all in the same direction, questions:

• Where should I start so as to implement my to-do?
• Any tutorial, how-to-do guide, api, library, or online course?
• What is the proper way to implement above to-do, programmatically?

Note: You can check the screenshot, which is taken from mentioned website.

Answer 1

Do I correctly interpret your question:

You want to check whether SMTP server (from which some mail arrived for example) exists and you wish to check whether that mail exists as well.

To do so, you can do few things:

Solution 1:

ping the server, for instance mail.example.com,

if server responds, then check whether ports that are usually used by SMTP are opened: usually 25

if at least one of them is opened then you can say that mail server on this domain is running.

This solution has a problem that some servers won't respond to ping. But instead of pinging it, you can request the host name from DNS to see whether you'll get IP for a domain. Then this solution has a problem that a server might have been there yesterday but it is not today, and DNS returns the IP, but nobody is at the end of it.

This you can achieve by calling ping, hostname, netmap and similar tools using subprocess or by programing it all yourself from scratch with sockets.

You can, off course, open a socket to the domain, directly to port 25, but you'll have to specify timeout because it can happen that whole thing blocks because no answer is coming from server. Yes, pinging is an extra step, but it is faster in cases where server doesn't exist. It just sends ICMP packets, it doesn't try any handshakes. (I know from experience).

Solution 2:

More acceptable solution is to use smtplib just to try connecting to that server.

It will "attack" the specified domain asking on default port and sending an SMTP request. The exception will be raised if the server is not SMTP or if it doesn't exist. Timeout also should be set. Pinging will here also increase speed of checks, and no timeouts needed. You just assume that if server doesn't answer, it doesn't exist. (Hm, cleverness of this aproach is questionable)

If anything, a firewall, filter or something alike, doesn't allow you any of those checks, you can always have an updated lists of all mail servers (if it can be optained from any robot) stored locally or connect to a service like one you mentioned to consult it about the address.

Solution 3:

Last resort solution for checking if an address exists is to try using that SMTP server to send ourselves an e-mail. If thing doesn't work, then e-mail doesn.t exist.

Big problem!!!: Most SMTPs doesn't ask for login yes, but what if this one does? And, most SMTP servers allow you to send an e-mail from any address that doesn't even exist!

For e-mail verification, well, most SMTPs have that option disabled so, you wouldn't be able to see whether an e-mail exist or not.

To do e-mail checking for real, you will have to consult POP server as well (in fact it goes first), to see whether there is anyone that can receive an e-mail on this address.

Here is the code to do checking only using SMTP (as you asked) by using smtplib:

from smtplib import SMTP
from socket import timeout as ExcTimeout
import socket
import sys

addr2check = "something@somethingelse.woops"
# Note, if the server is subdomained for SMTP (like mail.somethingelse.woops) be sure to include it or server wouldn't be found!
#addr2check = "someone@mail.somewhere.here"

ok = 0
r = None
try:
    # For Python 2.5 and below (smtplib doesn't support timeout, so do it globally)
    if sys.version[:3]<="2.5":
        TO = socket.getdefaulttimeout()
        socket.setdefaulttimeout(8)
        try: s = SMTP(addr2check.split("@")[1])
        except: socket.setdefaulttimeout(TO); raise
        socket.setdefaulttimeout(TO)
    else: s = SMTP(addr2check.split("@")[1], timeout=8) # Maybe even bigger TO for some GSM and 56K modems
    ok += 1
    r = s.verify(addr2check)
    if r!=-1:
        ok += r[0]==250
    else:
        r = s.expn(addr2check)
        if r==-1: ok = -1
        else: ok += r[0]==250
    s.quit()
except ExcTimeout: print "Connection timed out! Server most probably doesn't exist!"
except Exception, e:
    # Here some checks for what exactly happened, or just:
    pass

msgs = {
    -1: "Server exists, but doesn't support e-mail verification!",
    0: "Server does not exist!!!",
    1: "Server exists but reports: "+str(r)+" for given e-mail!",
    2: "E-Mail "+addr2check+" most definitely exists! Confirmed by it's SMTP server!"
    }

print msgs[ok]

To extend your checking to POP3, use poplib! Algorithm will be similar to this one.