How do ISPs, ESPs, and blocklists communicate?

When you hit send, a lot of invisible conversations happen in milliseconds. ISPs, ESPs, and blocklists don't sit in a group chat, but they do talk to each other constantly, through a mix of automated systems and human relationships that most senders never think about.

Here's what actually happens when your email travels from your ESP to someone's inbox.

Step 1: Your ESP connects to the receiving ISP

Your ESP (Email Service Provider) hands your message to the receiving ISP (Internet Service Provider, meaning the company that runs the inbox like Gmail, Outlook, or Yahoo Mail) using SMTP (Simple Mail Transfer Protocol). That's the standard handshake that moves email between servers. Before the ISP even looks at your message content, it starts running checks.

Step 2: The blocklist check happens in real time

One of the first things a receiving server does is run a DNSBL query. DNSBL stands for DNS-Based Blocklist. Your sending IP address gets looked up against one or more blocklist databases in real time, using the same DNS system that translates domain names into IP addresses. If your IP shows up on a blocklist like Spamhaus, the receiving server gets that answer back in milliseconds and can reject the connection before your message is even transferred. That's why blocklist listings can cause immediate delivery failures with almost no warning.

Step 3: Authentication results pass through the message headers

Once the message is transferred, the receiving ISP checks your authentication signals, meaning SPF, DKIM, and DMARC. These aren't conversations between servers exactly. They're DNS lookups the receiving ISP runs on your domain to verify that your ESP was actually authorised to send on your behalf. DMARC reports (summaries of pass and fail results) flow back to you by email, usually daily, from every ISP that processes your messages.

Step 4: Feedback loops close the loop on complaints

When a recipient hits "This is spam," major ISPs don't just delete the complaint. They send it back to the ESP that delivered the message through what's called a feedback loop (FBL). The report arrives in a standardised format called ARF (Abuse Reporting Format), which ESPs can parse automatically. A well-run ESP uses those ARF reports to suppress the complaining address immediately so you stop sending to someone who doesn't want your email. If your ESP isn't processing FBL reports, your complaint rate quietly climbs and your reputation takes damage you won't see until it's too late.

Step 5: Human relationships handle what automation can't

All of the above is automated. But there's a parallel layer of human communication that matters just as much. Postmaster teams at Gmail, Outlook, and Yahoo maintain direct relationships with large ESPs and high-volume senders. Industry forums like M3AAWG (Messaging, Malware and Mobile Anti-Abuse Working Group) bring ISPs, ESPs, and blocklist operators together to coordinate on policy, share threat intelligence, and sort out emerging abuse patterns before they become systemic. Blocklist operators like Spamhaus run delisting portals and, for serious cases, direct contact processes for investigating listings.

So to summarise the two layers: the automated layer (DNSBL queries, ARF feedback reports, DMARC XML reports) runs continuously without any human touching it. The human layer (postmaster relationships, industry forums, blocklist delisting requests) exists to handle escalations, edge cases, and problems too complex for a script to resolve.

If something in this chain is breaking for you right now, our SOS hotline is free. No pitch, just help.