Resolving MX Records without delay (upgrade from ADSL -> Leased Line)

The best thing to do would be to lower the TTL (time to live) for the MX records to something like 300. Then let those changes propogate for the 72 hours. During this time everything should function like it normally did, but when you change the MX records (after the ttl has propagated) it'll only take 300 seconds (or whatever value you used) for the new MX record to fully propogate through the internet. Once the new records are updated you can change the ttl back to what it was.

Since the MX records have already been updated there really isnt much they can do. They can since the new records will have already started propagating. I would just let them go and make a note of what I mentioned above for the next customer. Not much email will be lost during this time, almost all email server will try for at least 5 days before returning undelivered mail.

The way I do this is to prepare well in advance.
Get a dynamic DNS address, they are free and add it to your existing MX records well before the switch, pointing at your existing MX records.
Once that has replicated, the DNS change is under your control.

Make the switch and then change the dynamic DNS address. As long as the original address has nothing on it, then the email should be delivered to the second MX record.
Get the original record updated with the new IP address.

Leave both running for a week or so, then get the dynamic DNS address removed.

Although for what its worth, I actually leave my clients with the dynamic DNS address in the MX on purpose. In the event of a failure I can get it switched to another IP very quickly.

Simon.

Thanks for both your answers - following on from what Simon posted - is it possible to have both the new and old MX records with different weightings then when propogated remove the old MX record and add the secondary MX record?

Thanks

Paul