Value Chain

No single company decides whether your email reaches the inbox. Several players each have a hand in it: you, the service that sends your email, the reader’s mailbox provider, blocklist operators and the reader. This article explains what each one does and what it wants, so you know where you have control.

Who is involved

You, the sender

A sender is anyone who sends email. Senders usually fall into four groups:

• Transactional senders send emails that a person’s action triggers: order confirmations, password resets, alerts. Shops, banks and apps all do this.
• Marketing senders send newsletters, offers and product news.
• Cold outreach senders email people who never signed up, such as sales teams and recruiters.
• Mixed senders do more than one of these.

Each group faces different risks:

Type of sender	Main challenge	Risk
Transactional	Must arrive at once. A late reset email loses trust.	Low: people expect and want it
Marketing	Must keep a large list interested	Medium: readers get bored, lists go stale
Cold outreach	No relationship. People often report it as spam.	High: unwanted by default
Mixed	Must keep marketing problems away from transactional mail	Depends on how well you separate them

You can’t control a provider’s filters. You do control what feeds them: who is on your list, your authentication, how much and how often you send, your content, and how you split your email into streams.

The ESP

An ESP (email service provider) is the company that sends email for you. Examples include Amazon SES, SendGrid, Mailgun, Postmark, Mailchimp and Brevo.

An ESP gives you:

• Mail servers and the IP addresses your email is sent from.
• Help setting up SPF and DKIM (records that prove your email is really from your domain).
• Bounce and complaint handling, so you stop emailing bad or unhappy addresses.
• Reports, templates and an API.
• Tools for warming up new IP addresses (raising volume slowly so providers learn to trust them).

Shared or dedicated IP. Your ESP decides which IP addresses your email leaves from. That choice matters.

• On a shared IP, your email goes out from the same addresses as many other customers. If one of them sends spam, your email can suffer too.
• A dedicated IP carries only your reputation. It needs enough steady volume to build that reputation, so ESPs usually suggest one only for senders with tens of thousands of emails a month or more.

Good ESPs watch their customers and remove spammers quickly. That protects everyone on their shared IPs. Cheap ESPs that tolerate spammers drag down the reputation of all their IP addresses. Picking an ESP is a deliverability decision, not only a price decision.

The mailbox provider

A mailbox provider hosts the reader’s inbox: Gmail, Microsoft (Outlook.com and Hotmail), Yahoo and AOL, iCloud Mail, Zoho, Proton and others. They are the gatekeepers. Their filters decide between the inbox, spam, or dropping your email.

They care about three things:

• Happy users. People leave if their inbox fills with spam. They also complain if wanted email goes missing.
• Safety. Blocking phishing, malware and scams comes first.
• Wanted mail. They want the inbox to hold email people actually read.

Each provider runs its own secret filters. They all look at:

1. Your reputation: your history of complaints, bounces and engagement.
2. The email: how it is built, its links and its attachments.
3. This reader: how they have treated your emails before.
4. Shared intelligence: blocklists, anti-spam groups and their own monitoring.

Providers share a little data with senders:

• Google Postmaster Tools shows your spam rate and authentication results at Gmail.
• Microsoft SNDS shows how your IP addresses behave at Outlook.com.
• Yahoo’s Complaint Feedback Loop sends you reports when Yahoo users mark your email as spam.

Each provider decides on its own. You can reach the inbox at Gmail and land in spam at Outlook, because each weighs signals differently.

Blocklist operators

A blocklist is a public list of IP addresses or domains that have sent spam. Mail servers check these lists when your email arrives. Well-known ones:

• Spamhaus: the most widely used. Its lists cover spam sources (SBL), hacked machines (XBL) and spammy domains (DBL).
• Barracuda: popular with company email filters.
• SpamCop: built from user spam reports.
• URIBL and SURBL: list domains that appear in links inside spam.

If you are listed, effects range from a higher spam score to a flat rejection. You can get listed for:

• Spam complaints from users.
• Hitting spam traps (addresses that exist only to catch senders with bad lists).
• Sudden, unusual jumps in volume.
• Reports from other anti-spam groups.

Getting off a list differs per list. Some need a removal request with proof you fixed the cause. Others expire on their own after a quiet period. Spamhaus listings can take time and effort to resolve.

Standards and industry groups

These groups set the rules and best practices everyone else follows:

• IETF (Internet Engineering Task Force): writes email standards, including SPF, DKIM, DMARC and ARC. DMARC is now RFC 9989, published in May 2026.
• M3AAWG: an industry group that publishes best practices against email abuse.
• The Spamhaus Project: shares threat data and runs the blocklists above.
• Validity (which bought Return Path): runs a sender certification program and inbox placement tools.
• DMARC.org: promotes DMARC and shares guides. It doesn’t write the standard.
• The BIMI Group: promotes BIMI, which shows your logo next to authenticated email.

They affect you indirectly, through the rules and tools that providers use.

The reader

The reader has the most power of all, often without knowing it. Every action sends a signal:

• Opening, clicking and replying raise your standing with that reader. Across many readers, they raise your reputation overall.
• Reporting spam is the most damaging signal you can get.
• Ignoring you slowly tells the provider your emails don’t matter.
• Moving your email out of spam tells the provider its filter was wrong.

Readers are also where your list comes from. How you got each address, and whether the person really signed up, sets the starting quality of the relationship.

How it fits together

Nobody sees everything

• You don’t know the providers’ exact rules.
• Providers don’t know if you are honest or a spammer.
• Readers don’t know why some emails land in spam.
• ESPs see patterns across customers, but not each reader’s inbox.

So you have to work it out from the few clues you get: bounce messages, complaint rates, engagement numbers and inbox placement tests. Mailwel’s test inbox is one such test.

Trust is the currency

Trust works the same way everywhere in this chain:

• Earned slowly through steady, good behaviour.
• Lost quickly through complaints, spam traps or abusive sending.
• Separate at each provider. Trust at Gmail doesn’t carry over to Outlook.
• Starts at zero for a new IP address or a new domain.
• Can differ by stream. Your receipts and your newsletter can earn different reputations.

Clean lists build trust for you. Strict ESPs build trust for their IPs. Engaged readers build trust for you with their provider. The feedback is often slow and indirect.

What each player wants

Player	Wants	Because of
Sender	Emails reach people who read them	Sales, sign-ups, keeping customers
ESP	Its customers reach the inbox	Happy customers and a good reputation
Mailbox provider	Users see only wanted, safe email	Keeping users, ad revenue, security
Blocklist	Correctly flags spam sources	Being trusted and used
Reader	An inbox with relevant, expected email	Their time, and trust in their provider

Problems start when these clash. A sender who chases volume emails people who don’t care, and gets complaints. An ESP that tolerates spammers hurts all its customers. A blocklist that lists the wrong servers blocks honest senders.

What to do next

1. Choose your ESP with care. Look at its IP reputation, how it handles abusive customers and its deliverability help, not only its price.
2. Separate your streams. Send transactional and marketing email from different subdomains, such as mail.shop.example and news.shop.example. Then a marketing problem can’t block your password resets.
3. Watch how providers see you. Use Google Postmaster Tools, Microsoft SNDS and a blocklist checker.
4. Only email people who want it. Send what they signed up for, as often as they expect.
5. Keep at it. No single setting guarantees the inbox. Check SPF, DKIM and DMARC with DNS lookup, and keep watching your results.
6. Think long term. Bought lists, blasting inactive readers and ignoring complaints cause damage that builds up. Clean sign-ups and regular list cleaning build trust that builds up.