EEmail Deliverability Glossary

ESP (Email Service Provider)

A platform providing email sending infrastructure and campaign tools — e.g., Mailchimp, SendGrid, Instantly, Lemlist. ESP choice affects your warm-up strategy.

An Email Service Provider is a platform that provides email sending infrastructure, campaign management, and deliverability tools. Examples: Mailchimp, HubSpot, Klaviyo, ActiveCampaign, SendGrid, Mailgun, Postmark, Instantly, Lemlist, Smartlead.

ESP choice and warm-up:

Some ESPs have shared IP pools with other senders. Their pool's reputation affects yours and vice versa.
Many cold email ESPs require or recommend warming up before high-volume sending.
Some ESPs have built-in warm-up features; third-party tools like Inboxwarm.ai offer more sophisticated, network-powered warm-up.

Important: Warm-up built into an ESP is warming up your sending reputation within that ESP's infrastructure. If you ever switch ESPs, you may need to warm up again on the new platform's infrastructure.

Frequently Asked Questions

Which ESP is best for cold email warm-up?

For cold email, purpose-built platforms like Instantly, Lemlist, Smartlead, and Woodpecker are designed around the warm-up workflow and integrate well with dedicated warm-up tools. They handle sending limits, bounce management, and unsubscribes automatically. For transactional or marketing email, Mailgun, Postmark, and SendGrid maintain well-managed shared IP pools that provide a good foundation. The best ESP for your use case depends on your target audience, sending volume, and whether you're primarily sending cold outreach, newsletters, or transactional messages.

What happens to my warm-up progress if I switch ESPs?

Your domain reputation (which lives with your domain in ISP databases) carries over to your new ESP. However, your IP reputation stays with the old IP addresses — your new ESP's IPs start with zero history from your sending. This means you'll need to re-warm the new IP infrastructure even if your domain reputation is strong. Plan ESP migrations during a low-volume period, ramp volume gradually on the new infrastructure, and run additional warm-up network activity during the transition to compensate for the cold IP period.

What is a shared IP pool and how does it affect deliverability?

A shared IP pool is a set of sending IPs that multiple customers of the same ESP use simultaneously. The pool's collective reputation is shaped by all senders using it — a few bad actors generating complaints or spam trap hits can degrade inbox placement for everyone on the pool. Major ESPs combat this with pool management: removing senders with poor metrics, maintaining separate pools for different sender categories, and monitoring pool health continuously. If you experience unexplained deliverability issues on a shared pool, ask your ESP for pool health data or request a move to a different pool segment.

Related Terms

Dedicated IP

An IP address used exclusively by a single sender. Gives full reputation control but requires complete warm-up from zero before high-volume sending.

Shared IP Pool

A set of IP addresses shared among multiple senders through an ESP. Cheaper than dedicated IPs but your reputation is partially affected by other senders' behavior.

Built-in Warm-Up (ESP)

Warm-up features built into cold email platforms like Lemlist, Instantly, or Smartlead. Convenient, but limited in network size and provider diversity vs. dedicated tools.

Email Infrastructure

The complete technical stack for sending and receiving email — servers, IPs, DNS, authentication protocols, and monitoring tools.

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