Cold email glossary
Email service provider (ESP)
An email service provider (ESP) is a platform that sends email on a business's behalf, most commonly a marketing platform built for opted-in subscriber lists. The term is also used loosely for inbox providers like Gmail and for any tool that sends email.
What is email service provider (esp)?
In its classic sense, an ESP is a marketing email platform: Mailchimp, Klaviyo, Brevo, and similar tools built to send newsletters, promotions, and lifecycle email to subscribers who opted in. They provide list management, templates, unsubscribe handling, and analytics, and they send from large pools of shared infrastructure whose reputation they protect carefully.
That protection is why nearly every marketing ESP prohibits cold email in its terms of service. Their acceptable use policies typically ban purchased lists and unsolicited outreach, because one customer's spam complaints damage deliverability for everyone sharing the infrastructure. Accounts that import cold lists usually get flagged and suspended quickly, and the bulk-sending format is poorly suited to cold outreach even while access lasts.
Cold outreach runs on a different stack. Instead of a bulk platform, it uses individual mailboxes on Google Workspace or Microsoft 365, set up on dedicated sending domains, driven by sequencing tools that send low volumes per inbox with human-like pacing. The receiving server sees one person emailing another, not a bulk blast, which is what the format requires.
The term gets muddied because people also call inbox providers like Gmail, Outlook, and Yahoo 'ESPs', especially in deliverability discussions. When someone says the ESPs are tightening requirements, they usually mean the mailbox providers that receive and filter mail, not the platforms that send it. Reading which side of the transaction is meant from context clears up most of the confusion.
Why it matters in cold email
Choosing the wrong category of tool is one of the fastest ways to burn a domain. Teams that load a cold list into a marketing ESP get suspended and inherit a reputation problem, while teams that push newsletter volume through cold outreach mailboxes hit walls just as fast. Cold email needs infrastructure designed for it: dedicated domains, individual authenticated mailboxes, throttled sending, and warmup, none of which a marketing ESP provides or permits.
How Sendful handles it
Sendful does not route cold email through a marketing ESP. The Outbound Engine provisions dedicated sending domains and individual mailboxes for each client, authenticates and warms them, and paces sending to stay within what mailbox providers tolerate. Your primary domain and your existing marketing stack stay untouched.
Can I send cold email with Mailchimp or another marketing ESP?
No. Marketing ESPs require recipient consent in their acceptable use policies and prohibit purchased or scraped lists, so cold campaigns typically get accounts suspended. Beyond the terms issue, bulk platforms are the wrong shape for cold outreach, which works best as low-volume sending from individual mailboxes on dedicated domains.
What is the difference between an ESP and an inbox provider?
An ESP sends mail on your behalf, while an inbox provider hosts mailboxes and receives mail. Mailchimp is an ESP, Gmail and Outlook are inbox providers, and Google Workspace sits in both worlds because its mailboxes both send and receive. In deliverability conversations the word ESP is often used loosely for inbox providers, so check which side of the send is meant.
What should I use instead of an ESP for cold email?
Mailboxes on Google Workspace or Microsoft 365, created on dedicated sending domains separate from your primary domain, with SPF, DKIM, and DMARC configured, warmed before use, and driven by a sequencing tool or a managed service that throttles per-inbox volume. The components matter more than any specific brand of software.
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