What Is ESP (Email Service Provider)?
An ESP (Email Service Provider) is a company that provides the infrastructure to send email at scale — handling delivery, authentication, and sending reputation on your behalf. ESPs range from mailbox providers like Google Workspace and Microsoft 365 to bulk-sending platforms like SendGrid, Amazon SES, Mailgun, and Postmark.
The term "ESP" is used broadly. In one sense it means any provider that operates email sending infrastructure — the servers, IP addresses, and authentication that get your mail delivered. In marketing contexts it often refers specifically to bulk-email platforms (SendGrid, Mailchimp, Amazon SES) that send newsletters and transactional mail through their own IP pools.
The most important distinction between ESPs is shared versus dedicated IP addresses. On a shared IP, your mail goes out from an address pooled with many other senders, so you inherit the collective reputation of that pool. On a dedicated IP, the address is yours alone, so you own — and must build and protect — its reputation entirely.
Which ESP you use shapes your deliverability. Reputation attaches to the sending IP and domain, so a poorly managed shared pool can drag down good senders, while a dedicated IP requires enough consistent volume to establish and maintain trust. For cold outreach, most senders use mailbox-provider ESPs (Google Workspace, Microsoft 365) on shared infrastructure and lean on domain reputation.
Shared vs dedicated IP addresses
On a shared IP, your outbound mail leaves from an address used by many of the ESP’s other customers. The upside is that established volume keeps the IP’s reputation warm without you needing to send heavily yourself; the downside is that a few bad actors in the pool can hurt everyone’s deliverability, and you have limited control. Shared IPs suit low-to-moderate senders and most cold-outreach mailboxes on Google Workspace or Microsoft 365.
A dedicated IP is used only by you. You get full control and full accountability: no one else can damage the reputation, but you must also build it from scratch and sustain enough consistent volume to keep it trusted. Providers generally recommend dedicated IPs only for high-volume senders (often tens of thousands of messages per day), because a dedicated IP with sporadic volume never establishes a stable reputation and can actually deliver worse than a well-run shared pool.
Types of ESP
Mailbox-provider ESPs (Google Workspace, Microsoft 365) give your team real inboxes and are what most cold-outreach and small-business senders use. Mail goes out on shared infrastructure, so domain reputation is the dominant signal. These are the ESPs warmup and outreach tools typically connect to.
Bulk/API ESPs (Amazon SES, SendGrid, Mailgun, Postmark) are built for programmatic and high-volume sending — transactional email, newsletters, and application mail — via API or SMTP relay, often with the option of dedicated IPs. Marketing ESPs (Mailchimp, Klaviyo, Brevo) add list management, templates, and campaign tooling on top of sending. Each type handles authentication and reputation differently, which is why the same message can perform differently depending on which ESP sends it.
Shared vs dedicated IP at a glance
| Factor | Shared IP | Dedicated IP |
|---|---|---|
| Reputation | Shared with the whole pool | Entirely your own |
| Control | Limited — provider manages it | Full — you own the outcome |
| Volume needed | Low to moderate | High and consistent (often 50k+/month) |
| Warmup | Pool is already warm | Must warm the IP from scratch |
| Best for | Cold outreach, small/mid senders | High-volume senders wanting control |
Frequently asked questions
What are ESPs?
ESPs (Email Service Providers) are companies that provide the infrastructure to send email — managing servers, IP addresses, authentication, and sending reputation on your behalf. They range from mailbox providers like Google Workspace and Microsoft 365 to bulk-sending platforms such as SendGrid, Amazon SES, Mailgun, and Mailchimp.
What is the difference between an ESP and a mailbox provider?
A mailbox provider (like Gmail or Outlook) operates the inbox that receives and stores mail for end users, while an ESP provides sending infrastructure. The terms overlap — Google Workspace and Microsoft 365 are both mailbox providers and ESPs — but bulk ESPs like SendGrid or Amazon SES focus on outbound sending rather than hosting inboxes.
What is the difference between a shared and dedicated IP?
A shared IP is pooled among many of an ESP’s customers, so you inherit the pool’s collective reputation but have little control. A dedicated IP is yours alone, giving full control and accountability but requiring consistent high volume to build and keep a trusted reputation. Shared suits most senders; dedicated suits high-volume programs.
Which ESP is best for cold email?
Most cold-email senders use mailbox-provider ESPs like Google Workspace or Microsoft 365, sending from real inboxes on shared infrastructure and relying on domain reputation. They send low daily volume per mailbox from separate outreach domains and warm each mailbox first, rather than using bulk ESPs designed for large marketing sends.
MailPilot connects to the ESPs and mailbox providers you already use — Google Workspace, Microsoft 365, and any SMTP/IMAP mailbox — and warms each one so you build the sender and domain reputation that deliverability depends on.
Connect your ESP and start warming up →