https://www.duocircle.com/email/email-backup-mx/
Backup MX: queueing secondary mail server with replay on recovery | DuoCircle
Secondary MX that queues incoming mail when your primary is down and replays it once recovery completes. Planned outages, unplanned outages, and tenant cutover...
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https://anydomain.net/setting-up-secondary-dns-for-mail-in-a-box/
Setting up Secondary DNS for Mail-in-a-Box - AnyDomain LLC
Oct 18, 2020 - Overview For every domain name (referred to as a ‘zone’ in DNS jargon) on the internet there needs to be a series of DNS servers known as a primary/master and...
in a boxsetting upsecondary dnsfor mail