Changing your Internet service provider (ISP) can be a big project. Because your Exchange server will receive a new IP address, you'll need to change your mail exchanger (MX) record in DNS.
DNS changes often concern mail administrators because these changes don't immediately propagate throughout the world. Some servers will receive the new address almost immediately, but others could continue to resolve to the old IP address for up to 48 hours.
When the destination server is unavailable, most mail servers queue mail and resend over a period of time that's sufficient for DNS changes to propagate everywhere. However, this can delay mail delivery. Fortunately, you can eliminate delayed mail with some forethought.
Normally, your new ISP will communicate your new IP address well in advance of any cutover. Once you receive the new IP addresses, determine what your Exchange server's new public IP address will be. Add a secondary MX record with the Exchange server's new IP address into your DNS, or ask your existing ISP to do it if it manages your DNS.
If you do this several days in advance of the cutover to the new service, both IP addresses will propagate everywhere before you make any changes. Now, any server sending SMTP mail to your company will try your existing IP address first, and then it will try the new IP address specified as the secondary MX record. This ensures mail delivery no matter which IP address your server is on. Your users, and the people that send them mail, won't notice a thing.
After you cut over to the new IP address, change DNS so that the Exchange server's new IP address is the primary MX record.