Your DNS records include a
TTL (Time To Live) field for each record. The TTL value specifies the requested time (in seconds) that DNS entries (such as MX) are cached. See:
https://en.wikipedia.org/wiki/Time_to_live
So if your old TTL value for the email related records (MX, SPF TXT, DMARC TXT) was 86,400 (one day), then over one day (maybe two days) before the changeover date you would change those TTL values to something small (such as 600 for 10 minutes). Then after the original TTL delay has passed (with some additional delay for good measure), you can change the DNS records (and their TTL values) to the new values.
Some servers may check your DNS records sooner than the TTL value specifies. But you can't depend on them using the new DNS record values until after the TTL-specified delay expires. This also depends on when the servers happened to have a need to check your DNS records. This is why some email servers may use the old MX value and others the new value.
Your sites should be:
- Examine your old TTL values and write them down.
- Change the TTL values of DNS records you will be changing to a small value (maybe 600). Be sure you verify that the authoritative DNS TTL values are actually changed by running a test.
- Wait a bit longer than the original (old) TTL delay time. After that delay, any check of your DNS records should indicate a small TTL, so your MX entry should not then be cached very long by servers following the rules.
- Change your DNS MX and other email-related records to the new desired values, including new TTL values (maybe an hour or a day).
But note this warning at the Wikipedia entry:
Quote:
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However, a problem persists in that some caching DNS nameservers set their own TTLs regardless of the authoritative records, thus it cannot be guaranteed that all downstream DNS servers have the new records after the TTL has expired.
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Also, some services accept different characters in the email address. Don't use any non-standard characters in your email address.
Bill