Mail servers of ledger.com.

A sending server looks up the MX records of the domain after the @ and tries the server with the lowest preference number first. Servers with the same number share the load. If none answers, the sender keeps the message and tries again later, usually for several days.

So a backup MX with a higher number only helps if it can actually deliver: a backup that accepts mail and then loses it is worse than none.

Without an MX record, senders fall back to the domain's own A or AAAA address and try to deliver there. A domain that should never receive mail says so with a "null MX": a single record 0 ., which tells senders to give up at once instead of retrying.

• It names a host, never an IP address.
• That host has its own A or AAAA record and is not an alias (CNAME).
• Every host listed actually accepts mail for the domain. A forgotten old provider still listed will bounce or swallow messages.

MX decides where mail to the domain goes. SPF and DMARC protect mail from it: SPF lists the servers allowed to send as the domain, and DMARC, at _dmarc. plus the domain, tells receivers what to do with mail that fails the checks.

Since 1 February 2024 Gmail asks every sender to set up SPF or DKIM, and senders of more than 5,000 messages a day to Gmail accounts to set up DMARC as well.