MX recored & Time to Live (TTL)
When you add or change an MX record, the TTL setting for the previously existing MX record determines how long it takes for every DNS server to start using the new MX record.
Shorter TTLs can cause heavier loads on an authoritative nameserver, but can be useful when changing the address of critical services like Web servers or MX records, and therefore are often lowered by the DNS administrator prior to a service being moved, in order to minimize disruptions.
When someone from another domain sends email to your domain, the domain name system (DNS) server for the sender’s domain contacts your domain’s DNS server for the prioritized list of MX records. The sender’s DNS server keeps a copy of the MX records so that it doesn’t need to contact your DNS server the next time it sends email to your domain. Each MX record includes a setting, called Time to Live (TTL), which tells the sender’s DNS server how long it can wait before contacting your domain’s DNS server to check for changes to the record.
When changing your MX records set a low TTL value so that you can quickly resolve any mistakes, or set a low TTL value for your existing records to facilitate the change.
The units used are seconds. An older common TTL value for DNS was 86400 seconds, which is 24 hours. A TTL value of 86400 would mean that, if a DNS record was changed on the authoritative nameserver, DNS servers around the world could still be showing the old value from their cache for up to 24 hours after the change.
Newer DNS methods that are part of a DR (Disaster Recovery) system may have some records deliberately set extremely low on TTL. For example a 300 second TTL would help key records expire in 5 minutes to help ensure these records are flushed world wide quickly. This gives administrators the ability to edit and update records in a timely manner. TTL values are “per record” and setting this value on specific records is normally honored automatically by all standard DNS systems world-wide.
