Question : 554 sorry, refused mailfrom because return MX does not exist

Don't blame me for the setup I'm describing below, its not my design.

I have a leased line with fixed IP address connected to an ISA server(win2000, ISA 2000) box, which is also a PDC, DNS, DHCP etc. etc. (call it MAINSERVER) with the fixed IP configured on the external NIC and the internal NIC attached to our LAN. On the LAN I have a WinXP box with IIS installed which I'm using as SMTP server (call it SMTP1).

Our website and mailboxes are hosted elsewhere by our ISP (lets call it ISP1). As it happens our LAN domain was configured to have exactly the same domain name that is registered with our ISP (once again, not my choice). So, on MAINSERVER I have an MX record defined in the DNS to point any mail to ourdomain.com to the "real" domain's IP that belongs to one of our ISP1's servers. Until a few weeks ago we also used a leased line provided by ISP1, but then we switched over to a leased line which is provided by another ISP (ISP2). Bear in mind the website and mailboxes are still with ISP1. So our external IP changed on the external NIC of MAINSERVER and this change happened fairly seamlessly. Until today that is...

Today the outgoing mail through our SMTP server stopped working altogether for any mail sent to our own domain (i.e. to [email protected]m it still works, but to [email protected] it doesn't). On SMTP1 the event log tells me 'message delivery failed to 'mydomain.com'. The connection was dropped by the remote host'. Helpful as that is, I figured since it was only happening on e-mails to ourdomain.com, it was likely a DNS problem, so I checked my MX record on MAINSERVER and after about ten calls to ISP1 decided to change it (they gave me a different IP to the one I had, but they deny that anything has changed so I just changed it to the new IP which they assured me was right). However, now all the outgoing mail is failing completely with "554 sorry, refused mailfrom because return MX does not exist"

As a workaround, I've tried changing the smtp server on all our mail clients to smtp.ourispsdomain.com, which worked at first, but now I'm getting 'relay not allowed from your location', so I guess I have to get ISP2 to allow me to relay SMTP to ISP1 or to use ISP2's smtp. This I should be able to sort out and I think I'll go for this solution anyway, since I don't really understand the reasoning behind having the internal smtp server when the mailboxes are hosted by the isp anyway.

The problem I have now is that in the day it took me to figure out all of this (I'm not really a network expert as you may have guessed by now...) the box SMTP1 has built up a queue of a couple of hundred messages. I need to get these out since some of them are/were quite urgent.

Any ideas would be much appreciated.

Cheers

Anton

Answer : 554 sorry, refused mailfrom because return MX does not exist

I've never actually used windows as an SMTP server, but other mail servers I've encountered have a config option to forward all outgoing mail to an upstream SMTP server.  If you can find such a setting on yours running on the XP box, then I would suggest pointing it at ISP2's outbount relay (most leased line providers offer an SMTP relay for use by ip's they own/control).  That should get everything delivered and eliminate the MX error messages.