The Impact of ISPs and ESPs on Cold Email Campaigns

When it comes to cold email success, crafting the perfect message is just the beginning. The unseen magic—or challenges—lie behind the scenes with your choice of Internet Service Provider (ISP) and Email Service Provider (ESP). These technical components can make or break your email deliverability. Mastering them can mean the difference between landing in the inbox and being relegated to the dreaded spam folder.

This guide will break down the roles ISPs and ESPs play in cold email campaigns, how they impact deliverability, and actionable tips to improve your email success rate. 

Table of Contents

1. What are ISPs and ESPs?

2. Types of ISPs and their impact

3. How ESPs optimize email success

4. Key deliverability factors (IP reputation, spam filtering)

5. Tools to improve email deliverability

6. Takeaways to improve your campaigns 

What are ISPs and ESPs?

ISPs (Internet Service Providers): Companies like Comcast, AT&T, and Verizon that provide you with access to the internet. Essentially, they handle the heavy lifting when sending and receiving data—including your emails.

ESPs (Email Service Providers): Platforms like Gmail, Outlook, or tools like Mailchimp or ConvertKit that offer the infrastructure for creating, managing, and sending emails. They also manage deliverability, automation, and performance analytics. Think of them as the bridge between you and your audience. 

How ISPs Impact Cold Email Campaigns

ISPs don’t just offer internet access—they play a huge role in the delivery of your emails. Here are two factors to pay attention to:

1. IP Reputation

An IP address is like your email’s home address. If the email associated with your IP shows spammy behavior, ISPs will flag it. Some ISPs use shared IPs, where multiple senders use the same address. The downside? Your emails could suffer because someone else using the same IP triggered a red flag.

Solution? Opt for dedicated IP addresses to ensure your campaigns remain unaffected by other users' actions. Yes, they are pricier, but they give you control over your IP reputation—an essential investment for high-volume campaigns.

2. Spam Filtering

ISPs use algorithms to detect spam emails. If your emails get flagged (maybe due to poor formatting, irrelevant content, or a shady IP), they won't reach the inbox.

How to avoid this?

• Use clear, clickable subject lines.

• Opt for well-designed email content.

• Always include an unsubscribe button (nothing says spam like no way out).

• Keep bounce rates low by maintaining a regularly cleaned email list.

How ESPs Optimize Cold Email Campaigns

If ISPs control access, ESPs control execution. They give you the tools to design and manage effective campaigns, but picking the right one makes all the difference.

Core Features ESSENTIAL for Cold Email Success:

• Sending Infrastructure 

A strong ESP maintains good relationships with ISPs, which helps emails bypass filters.

• Tracking Tools 

Delivery rates, open rates, and click-through rates—access to these metrics is key to understanding whether your campaigns perform or flop.

• Automation & Personalization 

Use dynamic fields to personalize messages and automate follow-ups to boost engagement (without sending one email at a time manually). 

Choosing an ESP optimized for cold outreach—like Mailshake or Mailchimp—ensures smooth campaign execution.

2 Critical Factors Impacting Email Deliverability with ISPs + ESPs

When running a cold email campaign, these two factors significantly affect whether your message hits the inbox.

1. IP Reputation (Again, It’s THAT Important)

We’re doubling down on IP reputation here because it plays into both ISPs and ESPs. Even WITH the perfect ESP, if your IP gets labeled as spammy, your emails will fail to land in inboxes. With dedicated IPs from reliable providers, you see higher deliverability rates instantly.

Bad IP Reputation Example? 

If you suddenly send 10,000+ emails from a brand-new IP, ISPs will flag it (almost guaranteed).

Pro Tip? Warm up new IPs by gradually increasing your sending volume over time.

2. Spam Filtering

Both the ISP and ESP's algorithms look for red flags, but usually for different reasons:

• ISPs have stricter filters when receiving emails.

• ESPs help clean up your emails so you don’t trigger those filters in the first place.

Tips to stay clear of spam filters: 

• A/B test subject lines (avoid excessive punctuation or words like “FREE!!!”). 

• Maintain list hygiene—engage only with active recipients. 

• Follow spam compliance laws (CAN-SPAM, GDPR). 

Tools That Save Your Email Deliverability

While ESPs provide some control, third-party tools truly elevate your deliverability. Here are your lifesavers:

1. Instantly: 

A robust tool for managing cold email campaigns. It optimizes deliverability, syncs easily with existing ESPs, and provides reputation tracking. 

Top Features? 

• Automated email warm-up tools 

• Reputation and spam scoring 

• Dynamic AI copy optimization 

Cost: Plans start at $37 per month. Go for the annual plan to save 20%! 

2. Litmus: 

Perfect for email design geeks, Litmus not only improves deliverability but ensures your campaigns look flawless across devices. 

Top Features? 

• Spam filter testing 

• Design previews in 90+ platforms 

• Centralized approval workflows 

Cost: Starting at $99 per month. 

These tools are essential to running campaigns that win (not crash and burn). 

Key Takeaways for Cold Email Marketers 

Here’s what you need to remember about ISPs, ESPs, and deliverability magic:

• Invest in a Dedicated IP: Shared IP addresses may save costs, but dedicated IPs save your campaigns.

• Pick the Right ESP: Not all ESPs are created equal. Choose one that supports personalized cold email strategies.

• Use Tools to Scale Smarter: Combine your communication platform with Litmus and Instantly for maximum efficiency. 

• Respect Spam Laws: They exist for a reason. 

• Always Monitor Campaign Data: High bounce or complaint rates? Tweak your emails before things go south!

Cold email success isn’t just about killer writing—it’s a mix of strategic tools, clean infrastructure, and constant refinement.