ISP Email Hosting stopped - Mail getting lost

Here's one that I just unraveled.  Perhaps it will save some of you some headache in the future.

 I've got a client that has SBS.  They are a fuse/zoomtown customer.  When I first got involved with them they did not have SBS and fuse was hosting their email.  When we implemented SBS we initially were using the POP mail connector.  Everything was working fine. 

Recently I wanted to switch their mail to be processed by Exchange Defender.  In order to take advantage of ED we needed to get of the POP Connector and get on to SMTP.  Fuse also hosted this company's DNS.  I contacted Fuse network operations and requested they change the MX record in the DNS from FUSE to EXCHANGEDEFENDER.  They made this change promptly and everything was working well – or so I thought.  On a recent office visit one of the staff at this company told me that some emails being sent to them were not being received.  A second staff member heard this and piped in – 'yeah, me too – some people have told me they sent me email and I never received them'.  I asked if they recalled the various senders that had reported sending 'lost' email and they remembered each one.  When we looked at the senders email addresses they were all @ email addresses.  I called fuse/zt tech support and spoke to someone there and explained the situation and asked if they could check this out.  They did something and then told me everything was fine on their end – 'must be something wrong with your exchange server or your Exchange Defender service'.  Today I had a revelation – one of those mental light bulbs that comes on in the shower, on the can, etc. and I figured it out.  I got someone else at fuse/zt on the phone and I told them what the answer was and asked them to dig in their system and confirm it.  Seems that when they changed the mx record to direct mail to a different destination – that was all they did.  They did not update ANYTHING in their mail server.  My client's domain and defunct pop mail boxes still existed in their server.  When the outside world sent email to my client their outbound mail servers did the proper thing and queried DNS and saw that this mail was supposed to go to EXCHANGEDEFENDER and routed it accordingly.  HOWEVER, if a FUSE/ZT customer initiated email – the fuse smtp server did not look at DNS first, it first looked at it's internal mail server and saw that there was a domain and mailboxes sitting right there in house.  They proceded to send emails that originated on the fuse network to these defunct mailboxes.  Voila – or wahla as I've seen.  This mystery solved.

Right now I am still waiting for cinbell to complete the fix.  Their first instruction to me was that I should call the business office and request they cancel our account.  I said 'that's not going to happen'.  When asked 'why not' I said that I KNEW that if I did this then certain other things – like our ISP service, the web site that they host for us would be guaranteed to disappear and we would have a Chinese firedrill ( no political incorrectness intended here) would ensue realtime during business hours.  So far they have deleted the individual mailboxes in their system but not the domain from the mail system.  So now, senders instead of getting no response when they were sending email to the dead letter mailboxes – get an immediate 'bad email address' because fuse still checks the internal mail server and sees there is no longer an individual box for the particular addressee.  I just have to get them to delete the entire domain from their mail server now so they route the mail properly to its correct destination at ED.