Advanced IP Segmentation and Allocation

If you send more than one kind of mail, the way you divide it across IP addresses decides whether trouble with one kind can damage the rest. Two questions shape the plan:

1. Which mail needs a reputation of its own, and so its own IP addresses?
2. How much volume does each group send?

When all of your mail is one kind and nothing needs to be kept apart, Basic IP Allocation is enough.

Assumptions

The advice here aims to deliver mail that recipients want and expect. Applied to mail that recipients do not want or expect, it can lower delivery instead, depending on the circumstances.

The four steps

1. Give transactional mail an IP address of its own, apart from marketing mail.
2. List the Mail Streams you send.
3. Combine or divide those streams into Reputation Realms.
4. Set the number of IP addresses for each realm from its volume.

Two terms

• Mail Stream: a category of mail that differs from your other mail in both its content and its recipient list. An email service provider (ESP) has one stream per customer. A company whose business units send unrelated mail to unrelated lists has one stream per unit. Most senders have a single stream, such as their newsletter.
• Reputation Realm: a set of IP addresses, domain names or both whose reputation is kept apart from every other realm. Realms exist so that mail in one realm cannot get mail in another realm blocked.

Step 1: Isolate transactional mail

Transactional mail is any message that a recipient triggers through their own action, such as an order confirmation or a password reset.

Recipients are waiting for these messages, so they are the best-delivered mail you send. On an IP address of its own, transactional mail is protected from problems that your other mail causes.

A dedicated address needs enough volume to sustain it. GreenArrow, a mail server vendor, puts the minimum at 1,000 messages per day. Below that, many senders route transactional mail through a well-run shared IP pool instead. The dedicated-IP volume floors table compares this floor with the general floor for any address and with the minimums ESPs set.

Step 2: Identify your Mail Streams

Usually there is only one. An ESP should treat each customer as a separate stream.

To decide whether two kinds of mail are really separate streams, picture how an outsider would see them. A spam filter, or a researcher studying your mail, should find a clear difference. If they would conclude that it is the same mail in a different wrapper, it is one stream.

Step 3: Form Reputation Realms

No rule settles this step. It takes judgment.

What a realm isolates

A realm works like a firewall. When one group of mail runs into trouble, the other groups keep their own reputation and their mail keeps arriving.

Separate IP addresses are the usual way to build that firewall. Receivers attach reputation to more than the sending address, though. For stronger separation, also give each realm its own value for the items below, which run from most to least important:

1. The domain in your open and click tracking links
2. The IP address that the tracking domain points to
3. The Return-Path domain
4. The domain in the reverse DNS names of your sending IP addresses. Change this one only when you need the strongest possible separation. Many different domains in reverse DNS resemble "domain snowshoeing", a spammer technique, and mail that merely looks like spam can be blocked.

Changing items 1 and 2 alone makes the firewall much stronger.

Separate mail by quality

Many senders build a "high quality" realm and a "low quality" realm, or several realms in tiers of quality. Quality here combines:

• Permission: stricter practices, such as confirmed opt-in, rank higher.
• Engagement: more inbox opens, and more subscribers who engaged recently, rank higher.
• Complaints: a lower complaint rate ranks higher.
• Content: subjects common in spam, such as weight loss, stock tips and online pharmacies, rank lower.

Keeping mail of similar quality together stops weaker mail from getting stronger mail blocked. Depending on the circumstances, more of your mail can then reach the inbox than if it all shared one realm. For an ESP, it means customers with poor practices cannot get customers with good practices blocked. The M3AAWG Sender Best Common Practices (Version 4.0, August 2026) make the same point for shared IP addresses: senders who share a pool should have similar content, complaint rates and bounce rates.

Mix mail of different quality in one realm

The opposite approach puts a large volume of strong mail and a smaller volume of weaker mail in the same realm.

When the proportions and the quality are right, the weaker mail is delivered better because of the stronger mail, and the stronger mail loses nothing. When the weaker mail is too poor, or makes up too large a share, delivery of the stronger mail drops. This approach works only with close monitoring and careful planning.

Divide one Mail Stream by quality

You can also split a single stream, with its better segments in one realm and its weaker segments in another. Common ways to split it:

• By where each subscriber's address was collected
• By recent activity, with people who recently opened, clicked or subscribed in the high-quality group

Use several realms for separation alone

Sometimes you cannot tell which streams are strong and which are weak. You can still spread them across several realms, so that your whole reputation does not rest on a single realm. If one stream then goes badly wrong, the damage stays inside its own realm.

Step 4: Size each realm by volume

GreenArrow sizes the number of IP addresses for a realm from its daily volume:

If you split a Mail Stream across several realms, the realms together must not use more IP addresses than the table gives for the stream as a whole.

Avoid too many addresses

Snowshoeing is a spam technique that spreads volume over many IP addresses, so that each address stays too small to notice. Mailbox providers look for it, and they penalize senders who use many addresses for the volume they send.

If you have been spreading mail this way, and your complaint rate is very high or your engagement is low, read Two Worlds of Email Deliverability before you move your mail onto fewer addresses.

Keep enough volume in each realm

Each Reputation Realm should send at least 40,000 messages a week. Below that, mailbox providers see too little of its mail to draw statistically significant conclusions about engagement and complaints.

Related articles

• Basic IP Allocation
• IP Warm-Up
• Two Worlds of Email Deliverability