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n5bb · 13 Aug 2018, 07:08 AM

First you need to look at your current published DNS records. Use MXToolbox.com or DNSstuff.com to look up your existing MX records. If you made a change to your MX records, they will propagate only after the previous TTL settings for that record time out. So what matters is the old TTL in your DNS records before the change.

Here are more details:

Domain records are cached. There is an upstream cache which is is queried when the TTL (Time To Live) value in the local cache times out. The TTL is specified in seconds in the raw database, so TTL=60 is one minute, TTL=3600 is one hour, and TTL=86400 is one day.
If the cached MX entry has timed out, an upstream provider is queried. Eventually the MX entry will time out at all levels and the authoritative DNS records will be queried.
Your DNS records (A records for websites, MX records for incoming mail, etc.) are located at a host specified by the NS record for your domain. You can point your domain NS records to Fastmail and host your DNS there. I do this, and find that the Fastmail hosting works very well. Fastmail automatically sets up DKIM and other features in your DNS records if they host your records. There is no cost for this service.
The new MX records will have their own TTL settings specifying how long that data should be cached.
Some systems ignore the TTL values and cache MX records for a longer interval. There is nothing we can do about that.

The best way to change your DNS records (MX, A, etc.) is:

If you are moving the DNS hosting to another host (such as Fastmail), write down your old NS TTL then change the NS TTL to a small temporary value (such as 60 seconds, which is 1 minute).
At the host you will use after the changes, examine and write down your old DNS entries (A, MX, etc.), including the TTL values. If you need to change your MX pointer, change your MX TTL to a small value (such as 60 seconds).
You then need to wait for the original TTL value to expire. In some cases, your TTL could have been set to greater than one day (greater than 86,400). You can’t assume that email will arrive at Fastmail until the old TTL expires and the caches are refreshed with the new MX entries.
The reason for the temporary short TTL setting is so you can fix any errors and test the fixes rapidly. You can then set your TTL values to a reasonable long-term value (such as 3600 seconds, which is one hour). The reason to use this larger value for normal operation is to prevent excessive DNS host queries and also to make DNS attacks more unlikely.

Bill

13 Aug 2018, 07:08 AM	#14
n5bb Intergalactic Postmaster Join Date: May 2004 Location: Irving, Texas Posts: 8,929	DNS and NS propagation after a change First you need to look at your current published DNS records. Use MXToolbox.com or DNSstuff.com to look up your existing MX records. If you made a change to your MX records, they will propagate only after the previous TTL settings for that record time out. So what matters is the old TTL in your DNS records before the change. Here are more details: Domain records are cached. There is an upstream cache which is is queried when the TTL (Time To Live) value in the local cache times out. The TTL is specified in seconds in the raw database, so TTL=60 is one minute, TTL=3600 is one hour, and TTL=86400 is one day. If the cached MX entry has timed out, an upstream provider is queried. Eventually the MX entry will time out at all levels and the authoritative DNS records will be queried. Your DNS records (A records for websites, MX records for incoming mail, etc.) are located at a host specified by the NS record for your domain. You can point your domain NS records to Fastmail and host your DNS there. I do this, and find that the Fastmail hosting works very well. Fastmail automatically sets up DKIM and other features in your DNS records if they host your records. There is no cost for this service. The new MX records will have their own TTL settings specifying how long that data should be cached. Some systems ignore the TTL values and cache MX records for a longer interval. There is nothing we can do about that. The best way to change your DNS records (MX, A, etc.) is: If you are moving the DNS hosting to another host (such as Fastmail), write down your old NS TTL then change the NS TTL to a small temporary value (such as 60 seconds, which is 1 minute). At the host you will use after the changes, examine and write down your old DNS entries (A, MX, etc.), including the TTL values. If you need to change your MX pointer, change your MX TTL to a small value (such as 60 seconds). You then need to wait for the original TTL value to expire. In some cases, your TTL could have been set to greater than one day (greater than 86,400). You can’t assume that email will arrive at Fastmail until the old TTL expires and the caches are refreshed with the new MX entries. The reason for the temporary short TTL setting is so you can fix any errors and test the fixes rapidly. You can then set your TTL values to a reasonable long-term value (such as 3600 seconds, which is one hour). The reason to use this larger value for normal operation is to prevent excessive DNS host queries and also to make DNS attacks more unlikely. Bill