How to Set Up a Custom Domain on Fastmail (DNS Walkthrough)

If you want to know how to set up a custom domain on Fastmail, the work is almost all DNS. Fastmail needs to receive mail for the domain, prove that it is allowed to send for the domain, and sign what it sends. That comes down to about seven records, or to one nameserver change if you let Fastmail host the DNS for you.

This walkthrough covers the domain side only: the records, the choice between Fastmail-hosted DNS and keeping DNS at your registrar, verification, and the two address features that depend on DNS (catch-all and subdomain addressing). Every record value below was checked against Fastmail's own help centre on 13 September 2026. Fastmail can change these values, so confirm them on its Manual DNS configuration page before you publish anything.

Which DNS records does Fastmail need for a custom domain?#

If you keep your DNS somewhere other than Fastmail, these are the records Fastmail publishes for an MX-only setup. Replace yourdomain.com with your domain. Fastmail's manual configuration page lists them all with a one-hour TTL.

What do you need before you start?#

Ten minutes of preparation saves most of the failures people hit later. The biggest one is losing website or verification records when nameservers change.

• A Fastmail plan that includes custom domains. Check Fastmail's current plan page, as packaging changes.
• A login for the place your DNS lives today. That is usually your registrar, or a DNS host such as Cloudflare.
• A copy of your existing DNS records, especially the A and CNAME records for your website and any TXT verification records for other services.
• A list of every service that sends mail as your domain: a newsletter tool, a CRM, a help desk, a billing system. Each needs to be covered by SPF and DKIM before you tighten DMARC.
• A decision on who hosts DNS: Fastmail or your current provider. The comparison table further down helps with that choice.

How do you set up a custom domain on Fastmail, step by step?#

The order below works for both DNS paths. Steps 3 to 6 only apply if you keep DNS outside Fastmail; if Fastmail hosts it, the wizard handles those records and you skip to verification.

1. 1

   1. Add the domain in Fastmail

   Go to Settings → Domains and run the domain setup wizard. Fastmail asks how you want to handle DNS and shows the values for your domain.

2. 2

   2. If Fastmail will host DNS, change the nameservers

   At your registrar, set the nameservers to ns1.messagingengine.com and ns2.messagingengine.com. Fastmail says these should be the only two NS records. Fastmail's help page does not say your old records carry over automatically, so re-create your website and verification records in Fastmail's DNS settings before or right after the switch.

3. 3

   3. If DNS stays elsewhere, add the two MX records

   Create MX records for in1-smtp.messagingengine.com (priority 10) and in2-smtp.messagingengine.com (priority 20), and remove every other MX record. Inbound mail starts moving to Fastmail as soon as these propagate.

4. 4

   4. Add or merge the SPF record

   If there is no SPF record, add a TXT record at the root with v=spf1 include:spf.messagingengine.com ?all. If one exists, add include:spf.messagingengine.com into it. Keep the total under RFC 7208's limit of 10 DNS-querying terms (include, a, mx, ptr, exists, redirect).

5. 5

   5. Add the three DKIM CNAMEs

   Create fm1._domainkey, fm2._domainkey and fm3._domainkey as CNAMEs pointing to fm1, fm2 and fm3.yourdomain.com.dkim.fmhosted.com. Fastmail holds the private keys behind those names. Many DNS panels add your domain to the host field for you, so type fm1._domainkey, not fm1._domainkey.yourdomain.com.

6. 6

   6. Add the DMARC record

   Create a TXT record at _dmarc with v=DMARC1; p=none;. That value asks receivers to take no special action, which is the safe start. Add a rua=mailto: address if you want aggregate reports, and only move to quarantine or reject once the reports show every legitimate sender passing.

7. 7

   7. Verify in Fastmail

   Go back to Settings → Domains and click Recheck DNS. Fastmail shows a green tick when it detects that your DNS entries are complete and correct. If a record is missing, the screen tells you which.

How long do Fastmail DNS changes take?#

Fastmail's own guidance is that propagation is normally a matter of minutes, but it may take up to a day, depending on the TTL of your previous records. A record that had a 24-hour TTL can keep old answers cached for up to 24 hours.

If you plan the switch in advance, lower the TTL on your existing MX and TXT records a day before you change them. Fastmail's records use a one-hour TTL, so later edits settle faster.

Should you let Fastmail host your DNS?#

Fastmail's help centre describes three setups. The practical difference is who edits the records and who catches mistakes.

How do you set up a catch-all or subdomain addressing?#

Both features depend on the domain working first, so do them after the green tick. A catch-all sends mail for any address at your domain to your account. Subdomain addressing lets you hand out addresses like [email protected].

1. 1

   Catch-all

   Go to Settings → My email addresses, click Add address, then Create an alias. Enter an asterisk in the address field, pick your domain so it reads *@yourdomain.com, and click Create alias. A specific alias always wins over the catch-all.

2. 2

   Subdomain addressing

   Go to Settings → Domains, click Edit next to your domain, then Manage in the Routing section. The default delivers [email protected] to [email protected]. Only one level works: [email protected] is fine, but deeper nesting is not.

3. 3

   Wildcard MX, if DNS is elsewhere

   Mail to a subdomain only reaches Fastmail if that subdomain has MX records. Fastmail's manual configuration lists a pair of *.yourdomain.com MX records for this. Fastmail-hosted DNS handles it for you.

What should you do when it doesn't work?#

Most failures come from one of a handful of causes. Check them in this order before contacting support.

Is there a faster way to handle what the domain receives?#

The DNS work above happens once, and Fastmail checks it for you. The part that repeats is the mail: a catch-all and subdomain addresses mean more addresses, more senders and more sorting every day.

We build AI Emaily, so weigh this accordingly. It does not host DNS, set up domains or replace Fastmail; your mail stays on Fastmail. It connects over IMAP with a Fastmail app password, triages what arrives at every address on the domain, and drafts replies in the voice you set in your Personal Context. Nothing sends until you approve it, and every action is logged and can be undone. The connect Fastmail guide covers setup, and Pro and Autopilot start with a 7-day free trial.