Custom Domains for Thousands of Agent Inboxes
Verify a dedicated subdomain on Developer+, then create agent mailboxes at scale. Workspace isolation, DNS checklist, and live Mermail plan limits.
By Toan Nhu
Verify a dedicated subdomain once, then create mailboxes in bulk on Mermail — so every agent can send and receive as [email protected] instead of a shared hosted domain.
TL;DR — Custom domains on Developer+. Use a dedicated subdomain with ≥3 labels (not bare apex). Verify DNS, then create mailboxes. Isolate tenants with workspaces.
Why agent fleets need their own domains
Shared hosted addresses are fine for prototypes. Production agents need domains you control: reputation lives at the domain, and one noisy outreach mailbox should not drag placement for billing or support. An agent inbox on your domain is the production pattern.
Plan fit (live pricing)
Custom domains are available on Developer and above. Free is for trying one mailbox — no custom domains.
| Plan | Price | Inboxes | Custom domains |
|---|---|---|---|
Free | $0 | 1 inbox, owner only | Not included |
Developer | $19/mo | 5 inboxes, 5 members | Included |
Growth | $49/mo | 10 inboxes, 10 members | Included |
Enterprise | Custom | Custom | Custom |
Numbers from mermail.app/pricing (2026-09-18): Free also includes 1,000 emails/mo and 1 GB storage; Developer 10,000 emails/mo and 10 GB; Growth 20,000 emails/mo and 20 GB.
Requirements before you start
Follow the custom domains guide.
- A Developer, Growth, or Enterprise workspace with custom domains enabled
- Workspace admin access
- Permission to edit DNS for the parent domain
- A dedicated subdomain with at least three labels — e.g.
mail.example.com. Mermail does not accept a bare apex likeexample.com.
Add and verify a domain
Choose the exact subdomain that should appear after @, add it in the Mermail console under the workspace’s custom-domain settings, publish every DNS record Mermail returns (copy type/name/value/priority exactly), then click Verify. Status moves from pending to verified when records check out.
Do not invent SPF, DKIM, MX, or DMARC values. Copy Mermail’s records. Some DNS UIs want the relative host; others want the FQDN — avoid doubling the parent zone.
Create mailboxes on the verified domain
Wait until status is verified, then create a mailbox and select that domain. Programmatically use Domains + Mailboxes in the API — or full-catalog MCP tools for add/verify domain and create_mailbox (Developer+).
# Conceptual flow — see docs for exact fields
# 1) add email domain → publish DNS → verify email domain
# 2) create_mailbox with an address on the verified subdomain
# Pass an idempotency key on create; list mailboxes before blind retriesA mailbox is the agent’s email identity. See the mailboxes concept.
Isolate tenants with workspaces
Mermail’s multi-tenant boundary is the workspace. Each workspace owns mailboxes, members, custom domains, billing, and usage. Put each customer in its own workspace with scoped API keys.
Use workspaces for tenant isolation. Mermail scopes resources to workspaces — one workspace per customer when you sell agent email as a product.
Fleet checklist
- Upgrade to Developer+ if you are still on Free
- Add a ≥3-label subdomain as workspace admin
- Publish every DNS record Mermail returns, then verify
- Create mailboxes (console, API, or MCP create_mailbox)
- Split workspaces per tenant when you sell agent email as a product
Wiring agents through MCP first? Start with How to Use MCP with Mermail’s Email API.
FAQ
Which plans include custom domains?
Custom domains on Developer+. Free has none. Growth and Enterprise also include custom domains — see mermail.app/pricing.
Can I use my bare apex domain?
No. Mermail requires a dedicated subdomain so you do not replace MX for existing company email.
