I have a strange problem that has been going on for a while now, and that is really beginning to confuse me. I have a client who has their domain registered with 123-reg, and it's forwarding to my web server. The client have a separate exchange server that is managed by a third-party company. Since their website was setup on my server in IIS, I have been unable to send any emails from my domains (also setup in IIS on the server) and theirs through my smtp server. However, if I use my ISP smtp server it works fine, but this is not always possible (ie. when I'm away and not connected through my ISP). If I send emails from hotmail, again it works fine. This isn't a massive problem as I can just use hotmail or use a different smtp server when possible, the big problem is with sending emails from the website, such as order notifications. These emails don't arrive, what we've had to do as a temporary fix (and this still doesn't seem all that reliable, but is much better than exchange) is to setup a mailbox on a different domain, send emails to that (which belongs to their third-part IT company, and is no way connected to my web server), and then that forwards onto the clients domain. This obviously isn't the ideal solution. To test this and try to replicate the problem I've setup my own Exchange server, and have directed one of my domains to that. If I send an email through outlook to that domain, it gets through to exchange fine (unlike sending to my clients domain). However, if I send an email through the website to that domain, it doesn't getthrough. If I send the same email to another domain with a pop3 mailbox setup on the web server, it works fine. I'm assuming the DNS is setup correctly because I can send email through outlook to my own domain, through my SMTP server,and it gets through fine (though as mentioned above, this doesn't work for the clients domain, even though dns is setup the same but pointing to a different exchange server). When I do try to send emails, I get a "message undeliverable" email back, saying that "sending failed due to invalid mailbox". However, the mailbox is valid because I can send to it through outlook. So basically I have 2 issues: 1) I can't send email from the website to exchange (my clients server or my own), but can to pop3 mailboxes 2) I can't send email from outlook through my smtp server to my clients exchange (but can to my own). To summarise the setup of the clients domain: - The domain is registered with 123-reg, forwarding to my web servers IP address (name servers not pointing to my web server). - The domain is setup in IIS on my web server for their website -DNS is setup on my web server to send email to the IP address of their exchange server (mx record is setup for this) I would appreciate any advice that anyone could offer on this matter. Thanks.

What I would add to this is that I'm wondering whether when sending from the websites its automatically trying to send to a local (ie. my web server) pop3 mailbox because the website exists in IIS, hence the "invalid mailbox" failure. Is there anything I need to do in IIS maybe to allow emails to be sent to Exchange? (I can't seem to stop SMTP for individual domain on my server, only for all of IIS).