I originally posted this in the "Web Programming" forum and in discussions over there it seems that the issue I may be having is related specifically to the Exchange server I'm trying to use. I'm going to re-post my messages over here in the hopes that someone will be able to tell me why I can't do what I'm trying and why there seems to be inconsistencies in the data that I've been finding. Sorry for the long post, but I'm just trying to post everything I already asked in the other forum and the only thing anyone was able to suggest what that it was some sort of network security issue and that I needed to ask someone who knows Exchange very well. Thanks in advanced for any help/suggestions/solutions that you can provide. This is something very important to an application I'm developing and without it I lose a major function.
Below is a combination of post I had written over a 2 day period based on additional information users were asking for. I cut it back some, but I just want to make sure I provide everything I can:
I'm a VB.NET developer and I like to expand by knowledge by trying to program things from scratch that are already implemented in the the.NET framework. My current project is programming a SMTP server using just sockets. I've already read the RFC on SMTP and my company has an Exchange Server (2007) that I was going to use for testing.
The web address of the server is exchange.mycompany.com and I used reverse DNS to get the IP adress. The domain name does have a MX record a well. From the command prompt on my computer (which is running Win7 Professional) I tried using telnet to connect on port 25. I tried using the domain name as well as the IP address, but both times it said it couldn't connect to the server on port 25.
I checked the settings in Outlook and the port is correct. I then tried connecting on port 110, and some other ones that are sometimes used, and it gave the same error message.
Next I connected to the Exchange server through VPN and tried telnet again on port 25 and this time it worked. However in the SMTP banner it said remote.mycompany.com instead of exchange.mycompany.com. So I disconnected from the VPN and tried the telnet again on all the ports I initially tried but used remote.mycompany.com but again got the same error message. I did an IP address lookup and remote.mycompany.com and exchange.mycompany.com point to the same IP address.
I then used mxtools.com and dnsstuff.com (I think that was the adress) and used their SMTP tools to see of they could connect. I entered mycompany.com as the domain name and they looked up the MX record, which returns exchange.mycompany.com as the SMTP server, and both tools were able to connect with no issues identified.
So my questions: 1) If the MX record list exchange.mycompany.com as the SMTP sever, why does the banner return remote.mycompany.com? 2) Why can't I connect the SMTP server through telnet unless I'm connected through VPN, but the two websites I used to test it the connection worked?
Obviously the server works because because the two websites can connect to it, I can when I'm in VPN, and other servers can because I receive email on my corporate account. What could prevent my computer from connecting to the SMTP server through telnet when I'm not on VPN. I should also mention that I even tried turning off all my anti-virus software to make sure that it wasn't blocking it.
I was thinking maybe because I don't have a domain name so if the Exchange server tries to lookup the IP address of my computer there is no MX or PTR record. Is this something SMTP servers look for?
Just today I also found something that I don't know how to explain or how it may affect this. The MX record for the server list exchange.mycompany.com. However, even when I'm on VPN, I can't connect to the SMTP server using that address in telnet or the IP address it points to. I have to use remote.mycompany.com, which is shown in the SMTP banner and points to the SAME IP ADDRESS. Is there a setting in Exchange that makes it script only request coming from this address? If so, how do the online SMTP tools discover this subdomain when I only gave the tools the domain name (mycompany.com) and the MX record only shows exchange.mycompany.com? The address remote.mycomany.com is not located anywhere in the zone file.
What I'm trying to do (at least for now) is figure out how to deliver mail to the Exchange server from my software. For example, if I build an SMTP server and a client which connects to my that SMTP server and I want to send an email to email@example.com (where the SMTP server for mycompany.com is hosted on the Exchange server), how would the SMTP server I created connect to the Exchange server in order to deliver the email? Doesn't the Exchange server have to allow some sort of connection from outside the intranet in order to receive emails from other domains (gmail, yahoo, etc.)?
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