Custom domains for email sending platforms
Sending records added without breaking existing mail. Your SPF include is merged into the customer's record, never duplicated.
For platform engineers, onboarding product managers and support leads at email platforms such as lemlist, Salesforge and Instantly.
Platforms that send as their customers
Each job with its records, its fit and the docs page behind it.
Send as each customer's domain
Notifications, receipts and campaigns go out from the customer's own address. Your include is merged into their SPF, never written as a second record. DKIM CNAMEs are checked. MX and SPF merges are written but not value-checked.
CNAME abc123._domainkey.mail.acme.com abc123.dkim.amazonses.comPartial fitDocs: Email DNS & SPF mergeTransactional email APIs
Receipts and password resets from a SaaS's sending domain. The DKIM selectors are per account, so your platform supplies them and each CNAME is checked against its value. MX records are not value-checked.
Email marketing for small businesses
Newsletters from a small business's own domain. The DKIM CNAMEs are checked, and a doubled host name such as acme.com.acme.com is flagged by the live check.
Commerce email and SMS
Branded sending subdomains for online stores. Static routing with CNAMEs and a TXT fits one supplied record set. NS delegation is refused in a connection and accepted only inside managed DNS zones, which are rolling out.
Lifecycle and product messaging
Onboarding and retention email from a SaaS's sending subdomain. The hourly drift check covers the records the connection holds. MX values are not value-checked.
CRM sending domains
Sales email from the CRM on a subdomain not used for hosting. Its DKIM CNAMEs are checked, and a CNAME collision is flagged before writing.
Help desk and chat email
Support replies from support@ on the company's domain. The DKIM CNAMEs are checked, and an SPF include that is still needed is merged in.
CNAME hd1._domainkey.acme.com dkim.example.netFull fitEnterprise engagement platforms
Regional or brand subdomains, each with link-branding and sending records. The API creates one connection per subdomain in bulk.
CNAME links.eu.acme.com links.example.netFull fitDocs: Create connections for a set of domains in one callAlso covered, with SPF merges and DKIM in a supplied set or one subdomain CNAME: event and community email, recruiting email, and preference and cancel pages.
Outbound and cold email
Many sending domains per customer, each with its own tracking subdomain.
Custom tracking domains
A tracking subdomain per sending domain, so each sender's links stop sharing one host. Deliver the vendor's CNAME as a supplied record, or serve the host over HTTPS on the edge on Growth and up.
CNAME track.acme.com tracking.example.netFull fitGrowth and upDocs: Where domains pointHow tracking domains connectCold email inbox infrastructure
Providers that set every record across dozens of domains per agency. Bulk create and a remembered provider key connect an account's domains together. MX and SPF merges are written but not value-checked.
White-label outbound for agencies
Lead generation agencies serving many clients under their own brand. The agency workspace adds client labels and setup links. Not every email record is value-checked.
Also covered, each one supplied tracking CNAME: sales engagement tracking and outreach tools that send from a mailbox.
Business email and inbound mail
Mail keeps arriving while the domain also points at a platform.
Mailbox suites on a customer's domain
Platforms that bundle email, and site builders whose customers add mail beside the site. MX, SPF as a merge, DKIM and DMARC go in one apply. MX is not value-checked.
MX acme.com 1 smtp.google.comPartial fitReply and inbound-parse subdomains
Help desks and CRMs that receive replies on reply.acme.com. A supplied MX on the subdomain is written but not value-checked.
MX reply.acme.com 10 inbound.example.netPartial fitEmail forwarding and aliases
A hello@ address on a domain without a mailbox. MX and an SPF merge can be supplied. CustomDomain™ has no forwarding product of its own.
MX acme.com 10 mx.example.netPartial fitMailboxes bundled with a domain
Site builders, registrars and MSPs that bundle mailboxes with a domain. The mail provider's TXT record is checked against its value. MX is not.
TXT acme.com v=spf1 include:_spf.google.com ~allPartial fitPrivacy and budget mail hosts
Mail hosts whose paid plans include custom domains, often many per account. The full set goes in one apply. MX is not value-checked.
MX acme.com 10 mail.example.netPartial fitSPF, DKIM and DMARC records
The record mechanics behind sending as a customer's domain.
Merge SPF, never add a second
A domain may publish only one SPF record. The SPF merge adds your include to the customer's record, deduplicated and keeping its qualifier, so policy never loosens. It is refused past 10 lookup mechanisms or 255 characters.
TXT acme.com v=spf1 include:mailgun.org include:_spf.google.com ~allFull fitDKIM keys too long for the panel
A 2048-bit DKIM key in a TXT record runs past 255 characters, and some DNS panels refuse or cut it. A DKIM CNAME that points at the sender avoids the problem.
CNAME sel1._domainkey.acme.com sel1.dkim.example.netPartial fitDKIM by CNAME delegation
Selector CNAMEs point at the sender, so keys rotate with no change in the customer's DNS. Supplied records ride the sign-in, API key and by-hand paths, and each CNAME is checked against its value.
CNAME selector1._domainkey.acme.com selector1.dkim.example.netFull fitDMARC beside your records
Publishing DMARC next to a platform's sending records. DMARC goes in as a TXT record, one per name. There is no policy merge or staged rollout.
TXT _dmarc.acme.com v=DMARC1; p=none; rua=mailto:dmarc@example.comPartial fitBulk-sender rules at large inboxes
Large senders need aligned SPF and DKIM plus a DMARC record. The email template groups MX, an SPF merge, a DKIM CNAME and DMARC for the widget's email screen. Alignment is not checked.
TXT _dmarc.acme.com v=DMARC1; p=none; rua=mailto:dmarc@example.comPartial fitSPF over its lookup budget
Several senders can push an SPF record past 10 lookups. The merge is refused past 10 lookup mechanisms or 255 characters, counting only the record's own mechanisms. Nested includes and void lookups are not counted.
TXT acme.com v=spf1 include:mailgun.org include:_spf.google.com ~allPartial fitWhere your customers' DNS is hosted
Six common providers and the way in each one offers.
| DNS provider | Way in today |
|---|---|
| Cloudflare | sign in |
| NameSilo | one-click |
| GoDaddy | api key |
| Amazon Route 53 | api key |
| Namecheap | by hand |
| Squarespace | by hand |
Common questions
Will connecting a sending domain break existing mail?
CustomDomain™ merges your SPF include into the customer's existing SPF record instead of adding a second one, which would break mail. Records already in the way are flagged before anything is written.
Which email records are checked against their values?
CNAME, A, AAAA and TXT records. MX, SPF merge and CAA records are written but not value-checked, so a set made only of those can go live without confirming them.
Does CustomDomain™ check DMARC alignment?
No. DMARC goes in as a TXT record you supply. Alignment, policy merges and staged rollouts are not checked or managed.
lemlist, Salesforge, Instantly, Cloudflare and other names on this page are trademarks of their owners. CustomDomain™ is not affiliated with or endorsed by them. Facts about other products were read from their public pages on October 30, 2026.
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