Mailboxes for Smartlead: Dedicated Domains and Workspace

Smartlead is popular with agencies because it handles many clients, many campaigns, and large mailbox counts in one place. That scale is exactly why the mailbox layer needs structure. When 200 mailboxes sit in one account, a single shared weakness (one domain carrying too many inboxes, one batch of fragile licences, one client sending a bad list) can quietly lower results across the board.

The fix is to design the infrastructure layer around isolation before you connect anything.

Per client domains. Every client gets their own set of sending domains. Never share a sending domain across clients.

Per client mailbox pools. Mailboxes on those domains only run that client's campaigns.

Separate from the agency's own domain. Your agency's company email never sends client cold email.

Separate from the client's primary domain. The client's real company domain stays out of cold campaigns.

Documented ownership. Decide up front whether domains are registered to the agency or the client, and write it into the contract. It matters on the day a client leaves.

1. Buy two to five domains per client depending on target volume.
2. Provision two or three Google Workspace mailboxes per domain to start.
3. Publish MX, SPF, DKIM, and DMARC for each domain. Check with a test send that all three pass.
4. Redirect each domain to the client's website.
5. Warm up roughly two to three weeks.
6. Add consistent sender names and signatures.
7. Connect to Smartlead, tag mailboxes by client, and set conservative daily limits.

Smartlead supports Google accounts via OAuth and via SMTP/IMAP with app passwords, and supports bulk upload for large batches. Practical tips:

Turn on 2FA before generating app passwords. Confirm IMAP access is enabled at the Workspace level. Use a consistent naming and tagging scheme so you can filter by client and by domain. Keep a source of truth sheet: client, domain, mailbox, warmup start, daily cap, status.

Daily caps per mailbox. Start low, raise gradually over weeks. Scale campaign volume by adding mailboxes on more domains, not by pushing each mailbox harder.

Rotation across domains. Let campaigns spread across all of the client's domains.

Tracking. Open tracking adds a pixel; many agencies run without it and report on replies and meetings, which clients care about more anyway.

Plain first touches. Short, plain text, few or no links, no attachments.

List hygiene. Verify every list. Bounces damage the mailbox, and the mailbox is shared by every campaign on it.

Work through this order:

1. One client or all clients? All clients points to something shared: your sending pattern, a template, or a tool setting. One client points to that client's domains, list, or copy.
2. All of that client's domains or only some? Some domains points to infrastructure on those domains. Check DNS, density, age, and provider health.
3. Bounces up? List problem.
4. Mailboxes being disconnected? Account or licence issue.
5. Before buying a new batch of domains, check blacklists and authentication. If they are clean, the drop is often volume, list, or copy, and new domains will inherit the same problem.

Plan this on day one:

Pause campaigns and stop warmup on the client's mailboxes. Export the mailbox list and any reply history the client is owed. Transfer domains if they belong to the client, or retire them if they belong to the agency. Remove mailboxes from Smartlead so they do not get accidentally reused.

Good isolation makes offboarding a checklist, not a crisis.

ColdMail is the infrastructure layer under Smartlead. It provisions official Google Workspace mailboxes on dedicated domains, publishes SPF, DKIM, DMARC, and MX automatically, and gives an official admin panel per domain with 2FA, which maps cleanly onto the per client isolation model above. Agencies can run multiple organizations, and AI warmup and Gmail, Outlook, and Yahoo health indicators are available on top. Mailboxes are ready to connect in Instantly and Smartlead using the methods those tools support, and also work with other tools you already use.

ColdMail does not replace Smartlead. Your campaigns, client workspaces, and reporting stay where they are.

See it at coldmail.app.