Advanced IP Segmentation and Allocation
Segmenting mail streams into reputation realms — isolating transactional from marketing mail, grouping by quality, and sizing IP counts by volume.
There are two main considerations in determining how to use IP addresses and how many to use:
- Segmenting email onto different IP addresses to maintain separate reputations
- Volume
If you only have a single kind of email and no need to split mail onto different IP addresses, see Basic IP Allocation.
What this document assumes
These recommendations maximize deliverability of email that is wanted and expected. Deliverability of email that is not wanted and expected may actually suffer from following these recommendations, depending on the details.
Overview of the process
- Put transactional email on its own IP address to keep it separate from marketing email.
- Determine the Mail Streams you have.
- Group or split Mail Streams into Reputation Realms.
- Decide the number of IP addresses for each Reputation Realm based on its volume.
Key concepts
- A Mail Stream is a type of email with a substantive difference in both content and mailing list from other types. Examples: mail belonging to different customers (if you are an Email Service Provider), or mail from completely separate business units sending completely different email. The most common case is a single Mail Stream — "your email newsletter."
- A Reputation Realm is one or more IP addresses and/or domain names that maintain a reputation distinct from other Reputation Realms. Its purpose is to isolate the reputation of different kinds of email so that email in one realm does not get email in another realm blocked.
Step 1: Put transactional email on its own IP address
Transactional email includes order confirmations, password resets — anything sent in response to a specific action taken by the recipient.
These emails get the best deliverability because they are extremely relevant to recipients. Isolating them onto their own IP address prevents problems with other mail types from affecting your transactional email.
This requires enough volume to support a dedicated IP: at least 1,000 messages per day. (Below that, sending transactional mail through a well-managed shared IP pool is a common alternative.) This is the transactional-isolation floor; for how it relates to the general per-IP statistical floor and vendor minimums, see the canonical dedicated-IP volume floors table.
Step 2: Determine your Mail Streams
Most senders have a single Mail Stream. If you are an Email Service Provider, consider each of your clients a separate Mail Stream.
When judging whether email qualifies as a separate Mail Stream, look at it from the perspective of an external observer: an anti-spam algorithm or researcher should clearly see a difference and not conclude "this is really just the same thing with different window dressing."
Step 3: Group or split Mail Streams into Reputation Realms
This is the step that requires a judgement call.
How Reputation Realms work
A Reputation Realm acts as a "firewall" between groups of email so that a problem in one group does not affect delivery of the others. Each realm maintains its own distinct reputation.
The most common separation mechanism is different IP addresses per realm. However, reputation attaches to more than the sending IP, so for a stronger firewall you may also want to vary (in order of importance):
- The domain name used in click and open tracking URLs
- The IP address that the tracking domain resolves to
- The Return-Path domain name
- The domain used in the reverse DNS of the sending IPs — only for firewalls of extreme strength. Too many domain-name variations in reverse DNS can look like "domain snowshoeing," a spammer technique, and merely looking like a spammer can get your email blocked.
The reputation firewall can be improved significantly by varying just items 1 and 2.
Strategy: Segregate email by quality into different realms
A common strategy is to create "high quality" and "low quality" Reputation Realms (or several realms of different quality tiers).
"Quality" means things like:
- Level of permission — tighter permission practices like confirmed opt-in are better.
- Subscriber engagement — higher inbox open ratios and more recently engaged subscribers are better.
- Complaint ratio — lower is better.
- Content — content related to spam-heavy industries (weight loss, stock tips, online pharmaceuticals) is worse.
Grouping email of similar quality prevents low-quality email from getting high-quality email blocked. Depending on the details, more email can reach the Inbox than if all mail shared one realm. For an ESP, this keeps customers with low-quality practices from blocking the email of customers with high-quality practices.
Strategy: Blend high- and low-quality mail in one realm
The opposite strategy: blend a lot of high-quality email with a smaller amount of low-quality email in a single realm.
If the relative volumes and qualities are right, the low-quality email receives a deliverability boost from the high-quality email without hurting it. But if the low-quality email is bad enough, or its relative volume too high, the high-quality email's deliverability suffers. This requires careful monitoring and planning.
Strategy: Split one Mail Stream by quality across realms
Sometimes you'll want to break a single Mail Stream out by quality, placing high-quality and lower-quality segments into different realms. Ways to divide a Mail Stream:
- By the source of the subscription record
- Placing recent openers, clickers, and subscribers into a high-quality category
Strategy: Multiple realms purely for separation
Sometimes you don't know which Mail Streams are high- or low-quality. Mail Streams can be split across several Reputation Realms simply to avoid "too many eggs in one basket": if something goes badly wrong with one Mail Stream, the damage stays isolated to a single realm.
Step 4: Decide the number of IP addresses per realm by volume
| Messages per day | Number of IPs |
|---|---|
| less than 40,000 | One |
| between 40,000 and 200,000 | Two |
| more than 200,000 | Size for 100,000–1,000,000 messages per IP address per day |
When splitting a Mail Stream into multiple Reputation Realms, do not assign more total IP addresses than the table would specify for the unsplit stream.
Don't use too many IP addresses
Spreading mail over too many IP addresses looks like snowshoeing — a spamming technique that spreads volume over many IPs so each flies "under the radar." Mailbox providers detect this and penalize senders whose IP count is too high relative to volume.
If you have been snowshoeing and are sending email with a very high complaint ratio or low engagement, read Two Worlds of Email Deliverability before consolidating onto fewer IPs.
Have enough volume on each IP
A single Reputation Realm should not send less than 40,000 messages in a week. You need enough volume for mailbox providers to gather statistically significant engagement and complaint data on your email.