Cold Email Infrastructure for Lead Gen and Marketing Agencies
Updated 17 October 2026 · 7 min read
Short answer
Agencies running outbound for several clients should give every client its own set of sending domains and mailboxes, never shared between clients, so one client's list or offer cannot damage another client's deliverability. Size each client at about one warmed mailbox per 30 emails a day with 2–3 mailboxes per domain, keep warmed spares for new clients, and report per client on replies and meetings rather than opens. Cost is mostly mailboxes: at ReplyDak's Scale price of $2.69 per mailbox, a client running 20 mailboxes costs about $54 a month plus domains.
For an outbound or marketing agency, infrastructure is part of the product. Clients judge you on meetings booked, and that depends on mail reaching the inbox. The biggest structural decision is isolation: what happens to client B when client A's campaign goes wrong?
Rule 1: separate domains per client
| Approach | What happens when one campaign burns a domain | Verdict |
|---|---|---|
| Shared agency domains for all clients | Every client sending from those domains suffers | Avoid |
| Agency-branded domains per client | Only that client's mailboxes suffer, but prospects see your brand, not the client's | Only for agency's own outbound |
Client-branded lookalike domains per client (e.g. getclientco.com) | Only that client is affected; prospects see the client's brand | Recommended |
Client-branded lookalike domains should forward to the client's website, and the client's main domain should never be used for cold sends. Agree in writing who owns the sending domains and what happens to them if the contract ends. See protecting the main domain.
Sizing a client
| Client package | Emails per day | Mailboxes | Domains (3 per domain) | Spare mailboxes |
|---|---|---|---|---|
| Pilot | ~150 | 5 | 2 | 1–2 |
| Standard | ~450 | 15 | 5 | 2–3 |
| Large | ~900 | 30 | 10 | 3–5 |
These assume about 30 emails per warmed mailbox per day, including follow-ups. Keep 10–20% spare warmed mailboxes so you can rest a domain without missing the client's volume. Price packages with the cost calculator.
Running outbound for multiple clients
- 1
Onboard: agree domains, ownership and offer
Pick 2–10 lookalike domains per client depending on volume, confirm who pays for and owns them, get the client's sign-off on sender names and signatures, and collect their postal address for the email footer where required.
- 2
Set up DNS and mailboxes per client
Each domain needs MX, SPF, DKIM and DMARC and forwarding to the client's site. Create mailboxes with real sender names approved by the client. Setup guide.
- 3
Warm up before the campaign start date
Plan about three weeks of warmup before the first cold send. Tell new clients this up front; most onboarding delays come from skipping it. Warmup guide.
- 4
Keep clients separate in the sequencer
Use separate workspaces, tags or campaigns per client so mailboxes never mix. A client's campaigns should only rotate through that client's mailboxes. Connect mailboxes to Instantly or Smartlead.
- 5
Verify every list, per client
Lists are the biggest risk you inherit from clients. Verify before sending, keep a suppression list per client (and honour opt-outs across that client's campaigns), and refuse purchased lists of personal email addresses.
- 6
Scale by adding mailboxes, not volume per mailbox
When a client wants more meetings, add warmed mailboxes and domains instead of raising daily caps. How many mailboxes per domain.
- 7
Monitor health weekly and rotate
Review bounce rate, reply rate and complaints per domain. Pause and rest any domain that degrades and bring in a warmed spare. Retire domains that do not recover.
Reporting clients understand
- Lead metrics: prospects contacted, positive replies, meetings booked, and per 100 prospects for each.
- Health metrics: bounce rate and spam complaints per domain, mailboxes active vs. resting.
- Avoid headline open rates: privacy features and tracking settings make them unreliable, and many teams turn open tracking off for deliverability.
- Show the infrastructure cost per client (mailboxes + domains) so pricing conversations are transparent.
Pricing infrastructure into your retainer
Infrastructure is a predictable monthly cost per client, so many agencies list it as its own line or build it into the retainer. Work out mailboxes and domains for each package, add a margin for spare mailboxes and domains you rotate in, and review it when a client changes volume. Because mailboxes take about three weeks to warm up, a client who wants to double volume next month needs the extra mailboxes ordered now. See what cold email infrastructure costs.
Compliance across clients
Each client's campaigns must follow the law where its prospects are: for example CAN-SPAM in the US (accurate sender, honest subject, postal address, opt-out honoured within 10 business days) and GDPR/PECR in the EU and UK. Under CAN-SPAM, both the company whose product is promoted and the sender can be held responsible. This is general guidance, not legal advice; check local law and put responsibilities in your client contracts.
How ReplyDak helps agencies
ReplyDak automates the repetitive part of onboarding each client: it buys lookalike domains through Namecheap (or uses ones the client owns), sets up DNS records on Cloudflare automatically, forwards domains to the client's site, creates Google Workspace mailboxes, and runs warmup with automatic slow-down or pause if bounces, spam placement or complaints rise. Export each set of mailboxes as CSV for Instantly, Smartlead or your sequencer. Plans are monthly: $35 for 10 mailboxes, $87 for 30, $269 for 100; domains $13 per year. Agencies that refer clients can also join the 20% lifetime affiliate program. ReplyDak does not guarantee inbox placement.
Spin up separate, warmed domains and mailboxes for each client.
Domains, DNS, Google Workspace mailboxes and warmup. From $2.69 per mailbox ($35 for 10).
FAQ
Should an agency use separate domains for each client?
Yes. Shared domains mean one client's bad list or offer can push every client into spam. Give each client its own client-branded lookalike domains and mailboxes.
How many mailboxes does an agency need per client?
About one warmed mailbox per 30 emails a day. A client sending 450 emails a day needs roughly 15 mailboxes on 5 domains, plus a few spares.
Who should own the sending domains, the agency or the client?
Either can work, but agree it in the contract, including what happens when the engagement ends. Many clients prefer to own domains that carry their brand.
How long before a new client can start sending?
Plan about three weeks. Domains, DNS and mailboxes can be ready within a day, but mailboxes need roughly 21 days of warmup before cold campaigns.
Does ReplyDak have an affiliate program for agencies?
Yes, a 20% lifetime affiliate program for customers you refer.