SMTP Authentication (Sophos as smtp relay to internal Exchange server)

Hi The Bee,

just create a receive connector for the UTM which receives SMTP without authentication on your exchange server.

Configure the other receive connector(s) following your needs with authentication.

And finaly take a look into the concept of receive connectors within exchange. Example

I think that should give you the right direction.

Best

Alex

Hi The Bee,

you should create a public SPF record for your domain and enable Perform SPF check in Antispam settings at your UTM.

Regards

mod

Hi Alex and mod,

thanks for the reply!.

The question will be if the Sophos will be Authenticating any client that reaches the SMTP proxy? 

Because I guess it there's is a connection between the Sophos and the Exchange server that doesn't require SMTP AUTH (which I expect, if not no emails could be transferred through port 25...) wouldn't also all emails from any client on port 25 pass without SMTP Auth?

I think I'm confused.  If the only thing that can relay inbound emails to your mail server is the UTM, then you don't need to worry about auth in Exchange.  You don't want to allow users to authenticate to the SMTP Proxy - only allow the mail server to relay off of it.  Your Exchange and AD servers should be authenticating users that want to relay mail off the Exchange server.  Have I misunderstood your situation?

Cheers - Bob

Hi Bob,

BAlfson said:

You don't want to allow users to authenticate to the SMTP Proxy - only allow the mail server to relay off of it.

How can I do that? how can I tell the sophos to deny access to users trying to send emails through port 25 (or better to Authenticate them first) while letting actual email servers to reach the sophos?

What we have now is internet <-> Sophos <-> Exchange. 

Regards.

Basic Exchange setup with SMTP Proxy.

Cheers - Bob

The Bee said:

Right now is possible to send emails using SMTP port (25) from one or our domain's user accounts to any another of our domain's user accounts without authentication. Which is, of course to me, a very big security vulnerability. (I tested it with telnet from outside of our networks... like if I were an attacker).

Please read my post about SPF! This is one possible way that you can use!

https://en.wikipedia.org/wiki/Sender_Policy_Framework

Regards

mod

Most mail servers will accept mail on port 25. How else would I be able to send mail to one of your users?

Telneting to a mail server and entering a valid user with data etc and a mail from will result in the mail server accepting the mail (either from your domain user or a complete stranger)

Are you getting this mixed up with an open relay?

Hi Louis,

if you setup a SPF txt record in public dns for your own domain and setup your utm to check SPF records, no one can send an email from an external source with a sender address from your own mail domain. In SPF is defined which sender addresses are allowed to send mails for this mail domain.

regards

mod

Hi Mod,

yes I'm familiar with SPF, DKIM & DMARC etc. However, I think this thread has some crossed wires here.

The OP states that he can send mail on port 25 from outside to any of his users. I'd be surprised if he couldn't. Without further detail, my guess is the OP has telneted to his mail server, entered a valid recipient, the data and from a sender and the mail server has accepted this (as you would expect it to)

The above is not the same as "relaying" mail for the OP's domain. The mail server is simply accepting mail for a valid recipient by the sound of it.

Now, if the OP turned around and said that he could send mail VIA his mail server to another user outside of his domain (without authentication or other restrictions), then that is entirely a different matter as an open relay he would have.

But it's no big shock to be able to telnet to a mail server and send an email to a valid recipient.