Welcome to the EMD Forums!
Are you familiar with TTL (Time To Live)? This is why changes to your DNS records do not produce immediate changes uniformly:
https://en.wikipedia.org/wiki/Time_to_live
Each DNS record (such as the MX record which points to your incoming email server) has an associated TTL value. This specifies the number of seconds that the DNS record is cached. Small TTL values produce greater nameserver loads and make it easier for an attacker to spoof your DNS records, so the TTL is sometimes set very long (over one day, or over 86,400 seconds). If the TTL is set to 86,400 it will take one day for caches to update to the new MX address after you change it. Note that the old TTL value is cached all over the internet along with the old MX address, so reducing the TTL in your DNS records only has an effect after the old TTL interval times out.
The best way to move an MX record is usually given as follows:
- Examine you old MX record at your DNS host and note (write down) the old TTL value.
- Change only the TTL value of your MX record to 300 (5 minutes), but do not change the MX address yet.
- Wait for a little longer than the old TTL value you wrote down. Most caches should then have timed out, although some caches may have local settings which ignore the authoritative TTL. There is nothing you can do about local nameservers which don't follow the rules.
- Change only the MX address to the new value at your DNS host, but do not change the TTL value.
- Wait a little longer than 5 minutes. Your new MX address should now be used for anyone sending you mail.
- Change the TTL for the MX record to 3600 (one hour) or more. This reduces the nameserver load and makes it harder for an attacker to spoof your DNS records.
Bill