This came up in work today.

I know this works:
System A sends email via Exchange B which relays the message to Exchange C where recipient D has their account (assuming B is configured to accept relay requests from A).

Now say you needed to secure the connection between A and B and you can't use SMTPS or a VPN. Could A use a local SMTP server to relay the message via TLS to Exchange B which then relays to Exchange C? Effectively the local SMTP server is just proxying everything to B or is it only possible for an SMTP server to send straight to the final destination server?

Hope that makes sense to someone!

What versions of exchange? What's the basic setup? Are both exchange servers on the same domain? what connections / network exist between the servers?

System A - application hosted externally at 3rd party data centre on separate network.
Exchange B - Our 2003 Exchange server in our DC.
Exchange C - Clients 2003 Exchange server at their DC.

Public Internet between A, B and C.

Why can't it just go from A to C?

Magic Johnson said:

Why can't it just go from A to C?

Click to expand...

Can't spoof the from address (to avoid spam filter issues) and it has to come from @companyname.co.uk. No control over the domain records so can't add a SPF record. Also we can configure enforced TLS between B and C.

I'm by no means a Exchange expert but the answer I believe is Yes. The only food for thought would be what SMTP server to use on System A. It would need to be TLS capable but I don't see it being a problem.

I guess the choice would depend on what OS is running on System A.

I do something very similar using "hmailserver" on a Windows box however without TLS. I believe it's supported (it was on the development roadmap in 2009).

Yes, but I use/admin/resell Hosted Exchange to my clients.

Sounds possible but not sure in practise.

Dom

Thanks guys, sounds like in theory it could work but probably more hassle than just sticking a VPN in or just forgetting about using the company messaging system, buying a domain and going with a hosted Exchange solution which provides POP3 and SMTP relaying with TLS.