Our MX only points at one host, do we need a backup MX?
Large providers (M365, Google Workspace) already have multiple destinations behind one MX name, so a single MX record is enough. With self-hosted or a small provider with one destination, a second MX with higher preference (lower number = higher priority) is sensible.
Try this first
- 1Check current MX with dig mx yourdomain.com. One MX to M365 (yourdomain-com.mail.protection.outlook.com) or Google (aspmx.l.google.com) is normal and already redundant under the hood.
- 2Self-hosted (Postfix, Exchange on-prem)? Add a second MX with higher preference (higher number = lower priority). That backup MTA queues mail during primary outage.
- 3A backup MX must run the same anti-spam policy as the primary, or it becomes a spam entry point.
- 4Skip 'free backup MX' services from marginal parties. A backup MX without spam filtering is a gift to spammers.
- 5Test failover by briefly closing the primary and see whether mail is accepted by the secondary.
When to bring us in
Hosting mail on your own iron with one hop isn't redundancy, it's a SPOF. M365 or Google is more practical than building HA yourself.
See also
- Our emails land in spam for some recipientsAlmost always an SPF, DKIM, or DMARC setting that is wrong or missing, or a sender name that mimics a well-known brand.
- Someone reports receiving phishing emails "from us"Read: spoofing. Someone is abusing your sender name, not necessarily your actual mailbox.
- An email bounces (NDR): delivery failedThe NDR text usually states the exact reason. Reading it is step one.
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