Deliverability
Updated
MX record (Mail Exchanger)
Definition
An MX (Mail Exchanger) record is a DNS record that names the servers that accept incoming email for a domain, each with a priority number. Lower numbers are tried first.
How it works
To deliver to [email protected], a sending server queries the MX records of example.com, sorts them by priority (preference) and connects to the lowest number first, falling back to higher numbers if it cannot connect. Equal priorities share load.
Each MX target must be a hostname with an A or AAAA record, not an IP address and not a CNAME (RFC 5321, RFC 2181). If a domain has no MX record, senders fall back to its A record.
A domain that never receives mail can publish a null MX (MX 0 .), defined in RFC 7505, so senders fail immediately instead of retrying.
Example
Inbox mail goes to mx1, then mx2. The ses subdomain receives bounces and complaints for outgoing mail.
Why it matters
For senders, MX matters on the envelope sender (Return-Path) domain. That domain is where bounces go, and many receivers check that it can actually receive mail before accepting a message.
A wrong MX on your root domain stops your inbox from receiving mail. Sending tools should never ask you to replace it, only to add records on a dedicated subdomain.
Best practices
- Keep your root domain's MX pointed at your mailbox provider. Add sending records on a subdomain instead.
- Point MX at hostnames, never at IP addresses or CNAMEs.
- Use a null MX on domains that only send, or never send nor receive, to stop pointless retries.
- Check that the bounce subdomain your ESP asks for has exactly the MX they specify, with no leftover records stealing the name.
How Lumail handles MX record
Lumail never touches your root MX. It asks for an MX record on ses.your-domain.com, the custom MAIL FROM domain, pointing to the region-specific Amazon SES feedback endpoint with priority 10, plus an SPF record on the same name.
That lets Lumail receive bounces and complaints for your mail, suppress bad addresses and keep SPF aligned. The exact values are on your domain page.
Check a real message with the free mail tester or the spam tester.
Frequently asked questions
What does MX priority mean?
It is a preference number. Senders try the lowest number first and move to higher numbers only if the preferred server is unreachable. Records with the same number share traffic.
Do I need an MX record to send email?
Not on the From domain itself, but the envelope sender (Return-Path) domain should be able to receive bounces. That is why ESPs ask for an MX on a bounce subdomain.
Can an MX record point to a CNAME?
No. RFC 2181 and RFC 5321 require the MX target to be a hostname with its own A or AAAA record. Some providers tolerate CNAMEs, but it is not standards-compliant.
Related terms, tools and docs
- AuthenticationSPFSPF (Sender Policy Framework) is a DNS TXT record that lists which servers may send email using a domain in the envelope sender (Return-Path).
- DeliverabilityHard bounceA hard bounce is a permanent delivery failure: the receiving server rejected the email and retrying will not help.
- DeliverabilityEmail deliverabilityEmail deliverability is the ability of your emails to reach recipients' inboxes rather than bouncing or landing in spam.
- AuthenticationDMARCDMARC is a DNS TXT record at _dmarc.yourdomain.com that tells receivers what to do with mail that claims to be from your domain but fails authentication.
- Free toolSPF checkerLook up a domain's SPF record and count its DNS lookups.
- Free toolFree mail testerSend a real email and get an SPF, DKIM, DMARC and content report.
- DocsHow to fix MAIL FROMMX and SPF records on ses.your-domain.com.
- DocsHow to verify a sending domainEvery DNS record Lumail asks for, in order.
- DocsEmail domainsSend from your own domain instead of a shared one.
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