GreenArrow Email Software Documentation

Advanced IP Segmentation and Allocation

One big question that many people have when setting up a new email server is “how many IP addresses should I be using”.

There are two main considerations in determining how to use IP addresses and how many IP addresses to use:

  1. Segmenting email onto different IP addresses to maintain separate reputations
  2. Volume

Advanced advice

This document also provides advanced advice.

If you only have a single kind of email and no need to split mail onto different IP addresses to maintain different reputations, see the Basic IP Allocation article.

What this document assumes

This document makes recommendations for maximizing 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

The quick overview is:

(1) Put transactional email on its own IP address to keep it separate from marketing email

(2) Determine the Mail Streams you have

Figure out how many types of email you have which each have a substantive difference in both the content and the mailing list from each other. Each of these are a Mail Stream.

For example, different Mail Streams can be:

  • mail belonging to different customers (if you are an Email Services Provider)
  • mail be completely separate business units in a company that are sending completely different email

The most common case is to have a single Mail Stream that is basically “your email newsletter.”

(3) Group or split Mail Streams into Reputation Realms

A Reputation Realm is one or more IP addresses and/or domain names that will have different reputations from other Reputation Realms. The purpose of a Reputation Realm is to isolate the reputation of different kinds of email from each other so that the email in one Reputation Realm does not get the email in a different Reputation Realm blocked.

It is typical to group Mail Streams into categories of likely quality and assign them to different Reputation Realms so that each Reputation Realm has mail of a similar quality.

It’s also common to split a Mail Stream into two sub-categories, one of higher quality and another of lower quality, and then assign each sub-category to a different Reputation Realm

(4) Decide on the number of IP addresses for each Reputation Realm based on its volume

Messages per day Number of IPs
less than 40,000 One
between 40,000 and 200,000 Two
more than 200,000 Between 100,000 and 1,000,000 messages per IP address per day

When splitting a Mail Stream into more than one Reputation Realm, make sure that you don’t assign more IP addresses than the above recommendations would specify if you did not split the Mail Stream into multiple Reputation Realms.

We don’t recommend having a Reputation Realm that sends less than 40,000 messages per week.

Detail on the steps

(1) Putting transactional email on its own IP address

Transactional email are things like order confirmations, password reset emails, or things that are a response to a specific action taken by the recipient.

These emails get the best deliverability because they are extremely relevant to the recipients. By isolating these onto their own IP address you can mitigate problems with other types of mail from effecting your transactional email.

However, this does require having enough volume to support their own IP address. We recommend a volume of at least 1,000 messages per day.

(If GreenArrow Email is hosting your email server, we may be able to send your transactional email through a shared IP pool.)

(2) Determine the Mail Streams you have

Most GreenArrow Email customers only have a single Mail Stream.

If you are an Email Service Provider, then you should consider each of your clients to be different Mail Streams.

Most other other businesses have a single Mail Stream.

When considering if email has substantially different content and mailing lists and qualifies to be a separate Mail Stream, you need to look at your email from the perspective of an external observer. For email to belong to a different Mail Stream, it should be such that an anti-spam algorithm or an anti-spam researcher will clearly see a difference and not come to the conclusion “this is really just the same thing with different window dressing.”

(3) Group or split Mail Streams into Reputation Realms

This is the step of the process that requires a judgement call.

How Reputation Realms work

The purpose of a Reputation Realm is to have a kind of “firewall” between different groups of email so that a problem in one group will not effect the email delivery of the other groups. You want each realm to maintain its own distinct reputation.

The most common way to do this is by using different IP Addresses for each Reputation Realm. However, reputation is assigned to more than just the sending IP address, so for a stronger firewall of separation you may also want to differ:

  1. The domain name used in click and open tracking URLs (called the “URL Domain” or “Click & Open tracking domain” in the GreenArrow Software)
  2. The IP address of the domain name used in the click and open tracking URLs
  3. The Return Path domain name
  4. The domain name used in the reverse DNS of IP addresses. This is only required for firewalls of extreme strength. Having too many variations of domain names in reverse DNS can make it look like you are doing “domain snowshowing,” which is a spammer technique, and simply looking like a spammer can cause your email to be blocked).

(These are arranged in order of importance. The reputation firewall can be improved significantly by just varying items one and two on this list.)

Strategy: Segregating email by quality into different Reputation Realms

A common strategy is to create a “high quality email” and a “low quality email” Reputation Realms (or multiple Reputation Realms of different degrees of mail quality.)

By “quality” we mean things like:

  • The level of permission. (Tighter permission practices like confirmed opt-in are better.)
  • The level of subscriber engagement. (Higher inbox open ratios, subscribers that have engaged more recently are better.)
  • The complaint ratio. (Lower complaint ratios are better.)
  • The content. (Content relating to industries that have lots of spam are worse. Examples are weight loss, stock tips, and online pharmaceuticals.)

Grouping email of similar quality together prevents low-quality email from getting the high-quality email blocked. Depending on the details, more email can get delivered to the Inbox than if all of the email is combined into one Reputation Realm

(For an Email Service Provider this can keep customers who have low-quality email practices from blocking the email of customers with high-quality email practices, which is a good thing.)

Strategy: Blending high-quality and low-quality mail in a Reputation Realm

Another common strategy is to blend a lot of “high quality email” and a smaller amount of “low quality email” into a single Reputation Realm.

This is the opposite of the “segregate email by quality” strategy.

If the relative volumes and qualities of the email are right, then the low-quality email can receive a boost in deliverability from the high-quality email without hurting the deliverability of the high-quality email.

However, if the quality of the low-quality email is low enough or if the relative volume of the low-quality email is too much, then the the deliverability of the high-quality email can be hurt.

This requires careful monitoring and planning.

Strategy: Assigning mail of differing quality from a Mail Stream to different Reputation Realms

Sometimes you will want to break a single Mail Stream out by quality and put the high-quality email and the lower-quality email into different Reputation Realms.

This can sometimes help keep the low-quality email from hurting the deliverability of the high-quality email.

Some ways to divide a Mail Stream are as follows:

  • By the source of the subscription record
  • Adding recent openers, clickers, and subscribers into a high-quality category.

Strategy: Multiple Reputation Realms for separation.

Sometimes you don’t know which Mail Streams are high-quality and which are low-quality.

Sometimes Mail Streams are split to several Reputation Realms instead of being combined into a single reputation realm, not because of anything to do with the quality of the Mail Streams but rather just to avoid putting “too many eggs in one basket.”

By using multiple Reputation Realms if something horrible happens to a Mail Stream and it causes its Reputation Realm to have deliverability problems, the problems can be isolated to a single Reputation Realm.

(4) Decide on the number of IP addresses for each Reputation Realm based on its volume

Don’t use too many IP addresses

Spreading mail over too many IP addresses looks like a spamming technique called “snowshoeing.” Snowshoeing spreads a volume of mail over a lot of different IP addresses with the idea that each IP address can fly “under the radar” by having only a very small volume on each IP address. The problem with this is that the ISPs are aware of this technique and penalize senders when they see mail coming from too many IP addresses relative to the volume.

If you have been snowshoeing and are sending email with a very high complaint ratio or a low engagement ratio, then you should read the document Two Worlds of Email Deliverability before changing to use a smaller number of IP addresses.

Have enough volume on each IP

We recommend that a single Reputation Realm not send less than 40,000 messages in a week.

This is because you need to send enough email from an IP address so that the ISPs get enough engagement and complaint information on your email to be statistically significant.

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