SMTP directly to a host's MX record

Question

I've tried playing around in python to learn more about the smtp protocol. More precisely I'm been trying to send a mail straight to a host's smtp server, but with little success.

For example, say I want to send a mail to a gmail.com address, I lookup the mx record for gmail.com:

>> nslookup -type=MX gmail.com
gmail.com       MX preference = 40, mail exchanger = alt4.gmail-smtp-in.l.google.com
gmail.com       MX preference = 5, mail exchanger = gmail-smtp-in.l.google.com
gmail.com       MX preference = 10, mail exchanger = alt1.gmail-smtp-in.l.google.com

Then I do the following in python:

import smtplib
# Tried both port 465 and 587 (can't test port 25 since it's blocked by my ISP)
s = smtplib.SMTP("alt1.gmail-smtp-in.l.google.com", 587) 

I tried several, and for everyone but one I always got a:
"[Errno 10051] A socket operation was attempted to an unreachable network" or
"[Errno 10060] A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because conneted host has failed to respond" exception.

I read somewhere that some mail servers do a reverse lookup on your IP, and rejecting the connection if it hasn't got a domain. How do they do that?
I also read somewhere that many mail servers reject incoming mails from dynamic IP addresses (which I obviously have as a private customer to my ISP). How can they check if an IP address is dynamic or static?

Are these the reasons most servers seem to reject my connection? Or is there something more to it?

Answer 1

Um, your problem is exactly this:

# Tried both port 465 and 587 (can't test port 25 since it's blocked by my ISP)

Google's MX server is listening on port 25. If your ISP does not allow outgoing connections on this port, then you will not be able to send SMTP messages the way you are trying to do. You should get this sorted out with your ISP.

Regarding the rejection of messages, sending e-mail directly like this does increase the likelihood that it will be rejected or flagged as spam. Particularly if you set the "from" address to something that does not match the domain associated with your IP address, or if your SMTP client sends a mismatched domain in its EHLO message, or if the content of your message looks "spammy". The actual behavior will vary according to how each individual MX server has been configured.

Answer 2

Direct to MX email like you describe above will be blocked by Gmail's SMTP servers, with an error message "421-4.7.0", however many other SMTP severs, like MailEnable will allow Direct To MX.

The following website has source code for .NET and PHP for Direct to MX code, http://www.directtomx.com - you may be able to consume the webservice in python using SUDS.