Email IP allocation and warm-up

Differences between shared and dedicated IPs, and warm-up strategies

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Xtremepush email delivery platform

This guide only applies to projects using Xtremepush's email delivery platform.

This is not applicable if your project is using Amazon SES.

IP allocation refers to the IP address used when the Xtremepush email delivery platform sends email to the receiving inbox service provider (ISP). The IP address represents the source of the email for the delivery process.

Xtremepush owns and maintains its own pool of IPs used for sending emails. It's not possible to use IPs from another provider.

IP reputation

IP reputation is the perceived quality of email coming from a particular source IP by the various ISPs (eg. Google, Microsoft, etc.). Reputation has a direct impact on delivery (whether an email is accepted by the ISP or bounced as spam) and deliverability (if an accepted email is placed in the recipient's inbox, junk folder, or silently discarded).

Reputation is increased by using good sending practices, for example:

  • Valid SPF records – Xtremepush takes care of this automatically
  • Valid DKIM signature – Set up by the Support Team when you verify your from address
  • DMARC compliance – Emails sent by Xtremepush are always DMARC compliant because only from addresses with valid DKIM signatures are allowed (a DMARC policy is not needed to achieve DMARC compliance)
  • Use fully opted-in data – Xtremepush only work with clients who use opted-in data; bought lists are not accepted
  • Send targeted, relevant emails
  • Valid Return-Path / sender address – Xtremepush adds the correct Return-Path to automatically handle any bounces from the ISP
  • Being open about using a bulk email marketing tool – Xtremepush adds appropriate email headers to register itself and provide automated unsubscribe functionality where supported
  • Configured feedback loop – Xtremepush have registered all sending IPs with supported ISPs so that reports of abuse are handled automatically where possible

However, reputation will be quickly lowered if the ISP detects behaviour such as:

  • A high number of bounces in a short time period; this suggests poor data management
  • Not honouring bounces, requests to unsubscribe, or spam reports through feedback loops
  • Using deceitful practices like misleading subject lines
  • Many of their users reporting the same or similar emails as spam

ISPs can only judge an IP's reputation by the email received, so consistent sending from the IP is another factor in reputation. An IP's good reputation will be lost if no email is sent. This is one the key factors in choosing between shared or dedicated sending IPs.

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An estimate of IP reputation can be obtained from Return Path's Sender Score service. Keep in mind that Return Path only work with certain ISPs, so the figures are not complete.

Shared and dedicated IP

A shared IP (also known as shared range) refers to a set of multiple IP addresses which are shared between multiple clients. Particularly useful for low-volume or infrequent senders, it allows a shared reputation from shared good sending practices with consistent volume to benefit a sender who wouldn't send enough emails to achieve it on their own.

A Dedicated IP is an IP address that is only used for a single client. This may be a single brand, or multiple brands handled by the same marketing agency.

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As standard, all new Xtremepush clients will start their email sending using one of Xtremepush shared IP ranges.

This allows a steady way to build up or migrate volume to Xtremepush during onboarding, after which it is possible to analyse criteria to move to a dedicated IP.

Moving to a dedicated IP

A dedicated IP is usually desired when a client with very good sending practice does not want the possibility of their reputation to be affected by any other senders. It may also be important for some brands to be seen by ISPs to be the only brand using a specific IP. This is also a requirement for IP certification schemes like Return Path Certification.

A sender using a Dedicated IP should be sending a consistent volume to maintain reputation, so sends in excess of 10,000 per day are ideal.

This article from ReturnPath explains why a consistent volume is important for reputation.

Choosing an IP address

  • Pre-warmed IP: Has been previously used in a shared range to send email for a variety of clients, so has a good reputation. Requires little, if any, warm-up configuration, so it can be used immediately. Recommended for most clients.
  • Cold IP: Has not been previously used or may have been last used more than 6 months ago, so will have no reputation. This may be needed by clients who have to keep their brand separate from other senders, eg. a high-profile brand which is often targeted in phishing campaigns.

As a cold IP has no reputation, delivery can be very difficult. It's therefore important to consider the following choice of warm-up strategy:

IP warm-up

A client starting sending on a dedicated IP has a choice between two strategies:

  • Using shared IPs to assist. Xtremepush can route a percentage of email through the cold IP and send the rest through a shared range. Deliverability engineers will then adjust the percentage over a period of time in favour of the dedicated IP until 100% is sent via the dedicated IP.
  • Using only the dedicated IP address. Xtremepush will be configured to send 100% of email using the Dedicated IP, but a throttle is added to slow the rate at which emails are sent to the ISPs. Deliverability engineers will then adjust the send rate over a period of time until it can be lifted entirely.

There is no set rate for adjusting the IP configuration, as it is affected by the volumes sent, and the reputation gained with the ISP. Engineers will adjust the configuration based on a number of factors including: ISP delivery deferral rate, and public and private IP reputation metrics.

Defer to Shared IPs

Rules are put in place to control the flow of email sends to the big ISPs when deferrals are detected on dedicated IPs. Deferrals are a normal part of email delivery, where the ISP temporarily rejects delivery, (eg. due to high volume or capacity constraints). Deferrals are different from bounces, as the ISP should still accept delivery at a later time.

Senders using dedicated IPs must choose one of the following options:

  • Use deferral to shared IPs: When deferred, email may be routed to a deferral queue using a shared range where delivery speeds are automatically altered to suit
  • Don't use deferral: Emails will always stay in their queue using the dedicated IP address. This will cause delivery to be slower, as emails must wait to be accepted from the dedicated IP address. Recommended for clients using certification schemes to ensure all email is delivered through the registered IP addresses

Spam filters and IP blocklists

It's normal (but unfortunate) that recipients may click report spam in their email client when they get emails they no longer want to receive or don't remember, even though the opt-in was legitimate (and Xtremepush only works with fully opted-in data).

Understandably, inbox providers use these reports as signals to sender reputation. Major providers such as Google and Microsoft will manage their own junk algorithms, but smaller inbox providers (ISPs) will often defer to a third party mail filter such as MessageLabs and MailControl, or a blocklist provider, for example Spamhaus and SORBS. Spam filters have the ability to take multiple signals into account, and with their tighter integration into the mail sending process, can apply restrictions such as deferrals (slowing delivery), sending to a junk mailbox or quarantine. Unlike true spam filters, blocklists work solely against IP addresses, typically with a simple 'on-off' mechanism, so that an IP address is either deemed 'allowed' or 'blocked', with no gradual filtering.

As such, a blocklist's response to a spam report can only be to mark the IP address as blocked (usually after meeting a certain threshold of reports), which the ISP will then use to refuse any new mail delivery until the block is lifted. When reports are seen across multiple IP addresses in the same range, the lack of finer filtering means they can take a wide approach to trying to block further 'spam' to their users by blocking a range of IP addresses.

All of this behaviour is a normal part of Xtremepush's day-to-day responsibility as an email service provider. It's why we configure our mail system to automatically back off when we get deferrals from ISPs, retrying sends for up to 3 days, as well as our team taking manual action to submit unblocking requests to blocklist providers where available.

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Secrets

The inner working of spam filters (third party or major ISP) are never known and always evolving. If their algorithms were understood, then it would make it possible for spammers to circumvent them.

Most blocklist providers will show the status of an IP address and some will allow requests to delist an IP, but it is rare for them to clearly display their thresholds.


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