Found a SMTP server which host an email address but still get a Delivery Status code

Question

I want to send a mail to a user which email adress ends by @thinc.local, as I can't send this mail the usual way (for instance gmail discusses with Google MX server which doesn't recognize this address), I had to find a mail server which host this email address.

So I connected in SMTP (telnet IP_add 25) to different servers and I checked with VRFY or EXPN command if the email address was hosted by the server or not.

I had one server which responded "positively":

To VRFY henry:

252 <[e-mail]>

To EXPN henry:

250 <[e-mail]>

I know the IP_addr of the mail server AND the domain name (barry.thinc.local). So I tried to send an email to [e-mail] by adding this server to the outgoing SMTP server list in Thunderbird (I chose no security or authentication method, maybe this is a problem). But I still get a Delivery Status code message back.

I would like to have your opinion, because I'm feeling that I missed something.

P.S: I'm working in a training environment, so don't need to ask if I'm doing something illegal ;-)

Answer 1

What you seem to be missing is that .local names, by their very nature, are only known and visible inside your local network, thus there is no way that an "outside" mail server could ever resolve them properly, MX or no MX. (The .local domain is specifically reserved for this purpose.)

It's not just the name though – 99% of the time, these "local" names resolve to private IP addresses as well (e.g. 192.168.x.x or fe80::x or other ranges reserved by RFC 1918). So even if your chosen mail server could resolve the name, it still couldn't reach the actual address.

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Remember that such "private" addresses are duplicated across many networks. Your home has an 192.168.1.1, your neighbour's LAN has a different 192.168.1.1, and several billion homes also have their own 192.168.1.1's.

The same situation is with .local names – the same whatever.local might mean different things in different places. Even if you could find a SMTP server that accepted mail to .local addresses, you still couldn't tell where it actually delivered them.

In other words, mailing @thinc.local via Gmail is like writing "To: J. Smith" on a paper letter and trying to send it through New York Postal Office or something.

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So the short answer is, you must deliver the mail either directly (by connecting to thinc.local:25), or through a server that runs in the same LAN as the thinc.local computer is on.

Try the first option – a direct connection from your PC, either to the MX (well, I'm guessing the domain you mentioned is the MX for thinc.local) or to the domain itself:

telnet barry.thinc.local 25

telnet thinc.local 25

In fact, since you said you have the IP address, try connecting to that as well.

If that reaches the recipient, you're done.