How to blocklist a domain or email address
How-to guide Updated 12 July 2026
How do you blocklist a domain or email in cold email?
Tell Claude which domains and addresses to suppress: competitors, current customers, unsubscribes, do-not-contact accounts. It adds them to your HotHawk blocklist through the MCP server, so those people are excluded from every campaign you run, current and future, before a single email is queued.
No credit card required.
Before you start
You need a HotHawk account and the HotHawk MCP server connected to Claude. It takes a couple of minutes and no code. Full setup is on the MCP server page.
Step by step
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1
You ask Claude
Add these three competitor domains to my blocklist so we never email them.
Claude callsmcp__hothawk__blocklist_createtype domain entries 3 domainsThe domains go on the blocklist. Anyone at those companies is now excluded from every campaign, current and future, before a single email is queued.
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2
You ask Claude
Also block these 40 current-customer addresses from this CSV.
Claude callsmcp__hothawk__blocklist_createtype email entries 40 addressesClaude reads the file and adds each address. Your live customers stop getting cold outreach they should never have been on, which is the kind of mistake that costs you the relationship.
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3
You ask Claude
Show me everything on the blocklist so I can check it.
Claude callsmcp__hothawk__blocklist_createaction review return current entriesYou get the full suppression list back in the chat: the competitor domains, the customer emails, the do-not-contact addresses, all in one place so nothing slips through.
Suppression is deliverability, not admin
A blocklist looks like housekeeping. It is one of the most direct levers you have on your sending reputation. The quickest way to earn a spam complaint is to cold email someone who never wanted it: a customer who thought they were past being sold to, a competitor who reports you out of spite, a prospect who already asked to be left alone. Every one of those complaints tells mailbox providers your outreach is unwelcome, and that judgement sticks to the inboxes you send from.
So the blocklist is not the boring bit you do last. It is the guardrail you set before you send. Get it right and the whole system underneath, mailbox rotation, warmup on real Google and Microsoft inboxes, careful volume, keeps working in your favour. Get it wrong and one bad send can undo weeks of it.
The lists worth keeping
Most teams know they should suppress people. Few keep the list current, because it lives in someone's head or a stale spreadsheet. The ones that matter are the obvious ones once you name them: current customers, live deals sales is nursing, direct competitors, anyone who asked off, and your partner or investor domains. Emailing any of those cold is somewhere between awkward and relationship-ending.
With Claude you keep it current in a sentence. Paste three competitor domains and it blocks the lot. Hand it a CSV of your customer accounts and it adds all forty with blocklist_create. Block a single address to stop one person, or a whole domain to exclude everyone at a company at once.
Set once, honoured everywhere
The point of doing this in HotHawk rather than a spreadsheet is that the blocklist is enforced at send time, across every campaign. Add a domain now and it is excluded from tomorrow morning's sends, from the campaign you launch next week, from a colleague campaign in the same workspace. You do not re-check it per campaign.
Ask Claude to read the whole blocklist back whenever you want to audit it, and sync it from your CRM through the REST API if you would rather it stayed current on its own. Suppression handled properly is quiet, invisible and exactly the sort of thing that keeps your reputation clean while everyone else is wondering why their inboxes went cold.
The HotHawk tools behind this
Every action here is a real endpoint. Claude reaches it over the MCP server, and you can hit the same endpoints directly from the REST API if you would rather build it into your own stack.
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mcp__hothawk__blocklist_createAdd domains and addresses you must never send to
Frequently asked
What should I actually put on the blocklist?
The people you must never cold email: current customers, live deals sales is working, competitors, anyone who asked not to be contacted, and partner or investor domains. A single info@ typo aimed at a customer can undo months of goodwill, so this list is worth keeping current.
Can I block a whole domain, not just one address?
Yes. Block an address to stop one person, or a domain to exclude everyone at a company in one go. Domains are the quick way to suppress a competitor or a customer account without listing every mailbox by hand.
Why is suppression a deliverability thing?
Because the fastest way to earn a spam complaint is to email someone who never wanted to hear from you, or twice over. Complaints and unwanted sends tell mailbox providers your outreach is unwelcome, and that follows the reputation of the inboxes you send from. A tight blocklist keeps that clean.
Does the blocklist apply to campaigns I already have running?
Yes. Once an entry is on the blocklist it is honoured everywhere: live campaigns, drafts and anything you launch later. Add a domain today and it is excluded from every send after that without you touching a single campaign.
Do I need to write code to do this?
No. You tell Claude what to block, or hand it a CSV of addresses, and it adds them through the MCP server. The same blocklist endpoint is in the REST API if you want to sync suppressions from your CRM automatically.