Technical Considerations For IP Warmup


In response to the recent news regarding Goodmail closing its doors, Tom Sather at Return Path published a blog entry regarding IP warmup and the difference it can make for inbox placement.

Tom sums up the need for IP warmup well:

If you had talked to any email marketer 10 years ago and asked them how they dealt with blocks on their IP addresses, the answer would probably be the same: “We just switched IPs.” Not only was this an unfortunate, albeit effective, way to deal with blocks, it also became a common method used by spammers. They would simply send from one IP address for a very short time and then move on to another, either with IPs they owned or through hijacked computers controlled by botnets. Because of spammers’ behaviors, ISPs and email providers respond by temporarily blocking and limiting the amount of email a new IP address could send. ISPs now treat any new sending IP address like a dog on a short leash, and only extend the leash when the senders’ reputation is proven.

I personally have seen what can happen when senders try to send too much, too soon, with senders trying to send millions of messages on their first day using new IP addresses and finding themselves blacklisted in short order. For a reputable sender the key is to start sending slowly and gradually increase volume on new IP addresses until a proper sending reputation has been established.

A Clarification

Before I get into some technical advice I’d like to clarify one thing from the Return Path article. Regarding the shutdown of Goodmail Tom has this to say:

There are a couple of reasons you still might have to send from a new IP address, such as moving to a new ESP, moving to a new data provider, or moving off of Goodmail. Goodmail had a unique way of tokenizing their customers’ mail by relaying mail through their own IP addresses, and consequently their reputation. Therefore, once you stopped using Goodmail, your traffic now goes through your IPs, which hasn’t had any traffic in awhile, which means you’ll need to work on building up your sending reputation again.

This statement applies to any customer’s of Goodmail’s hosted imprinting service but does not apply to in-house senders using products such as Momentum by Message Systems that had a built-in Goodmail Imprinter. For such users the shutting down of Goodmail involved shutting off the Goodmail Imprinter component of their infrastructure but IP warmup will not be required since those users were already sending using their own IP addresses.

Tom’s Advice

Assuming you didn’t get a chance to read the link, here’s the five points of advice provided by the article:

  1. Sign up for all feedback loops. Suppress from future mailings.
  2. Authenticate. Use SPF, SenderID and DKIM.
  3. Segment and mail your active subscribers. Put your best foot forward.
  4. Monitor. Use seedlists such as Mailbox Monitor and watch your IP’s Sender Score.
  5. Get Certified. Get your new IP Sender Score Certified.

Some of these warrant additional discussion from a technical point of view.

Feedback Loops

In order to be effective, Feedback Loop message handling needs to be automatic. Message Systems customers should already be aware that we have provided built-in Feedback Loop processing as of our 3.0 release in 2008. In addition to automatically unsubscribing recipients that trigger a Feedback Loop message, you should also take the volume of feedback loop hits as a metric to show the effectiveness of your mailings. Feedback Loop hits should also be used as a factor when determining traffic shaping rules, especially when an IP address is new. If you see a lot of FBL hits, you should throttling back on the sending IP address.


With regards to monitoring, seedlist monitoring is a good indicator of how ISPs are treating your mail, but they provide only part of the overall picture. To get a complete view of your deliverability you need to also monitor what happens before the ISP accepts a given message, taking into account what temporary (aka transient or 4xx) failures and permanent (aka 5xx) failures that are occurring as you try to send. When monitoring permanent failures, keep in mind that permanent failures can occur both during delivery (synchronous or in-band) and post-delivery (asynchronous or out-of-band) through the ISP sending back a DSN (delivery status notification) message to the return path (aka envelope sender or envelope from) address of the original message. You should be tracking and trending all failures, especially when sending on new IP addresses.

Additional Technical Considerations

Keep in mind that just like in life, you are judged not just by what you say, but by how you say it (and whom you say it to). With regards to deliverability this comes down to content and sending practices. From a technical standpoint we focus less on content (what you say), but it does have a significant impact on IP warmup. You should avoid sending riskier content on new IP addresses both on overall content and wording (I’ve seen deliverability dip just for using the word “sexy” in a mailing, even when the overall message was not sexual in nature).

Sending Throttles

Before you ever start sending from a new IP it is vital that you pre-configure your sending software to comply with as many published ISP recommendations as possible. A convenient resource is this page provided by Word to the Wise: ISP Summary Information. Pay particular attention to the Connection Limits and Sending Limits columns.

Sending Volume

When warming up IP addresses it is important to start slowly; ISPs do not trust new IPs and will not respond well to new IPs coming online and immediately bursting out large amounts of traffic. While there are no published limits online one of the recommendations I have heard is to avoid sending more than 10,000 messages per day to the major ISPs (Yahoo!, Gmail, Hotmail, AOL, etc.) when first sending, and I’d say it would be best to send less than 1,000 messages per day to any smaller ISPs. By reviewing your temporary and permanent failure messages you will be able to get a feel on whether your reputation is sufficient to increase volume, and after increasing volume you should pay particularly close attention to your failure metrics to make sure that the change has not had an adverse effect on deliverability. I generally recommend not increasing volume by more than 2x at a time and not more than once every day or so. Don’t hesitate to revert to a lower volume if you start seeing an increase in temporary and permanent failures.

Suppress Bounces

When sending, make sure to quickly and automatically suppress any recipients that the ISPs identify as being invalid. When first sending you should assume that you are being watched closely, and one aspect of that is your practices regarding bounce processing. If you repeatedly send to someone that an ISP identifies as invalid through a bounce message you will be penalized for it by the ISP, and that punishment can potentially come faster when an IP address is new due to the lower starting reputation.

Watch Out For Deferrals

There is a specific class of temporary and permanent failure responses that you need to keep a particular eye out for, the deferral messages. A deferral message from an ISP indicates that you need to quickly and decisively change your sending practices as they are the warning messages you receive from the ISPs prior to being blacklisted. You can get examples of deferral messages from Yahoo here and many ISPs will list examples of their deferral messages on their postmaster page. As an example, here is a hotmail deferral message:

421 4.16.55 [TS01] Messages from x.x.x.x temporarily deferred due to excessive user complaints

When you see such messages, you need to review your content and throttles and pause sending for a couple of hours to allow things to cool off while you determine what changes you need to make.

Automating IP Warmup

In his article Tom Sather advised:

If this looks like a lot of work, then you’re right. To be successful, you need to plan appropriately, be patient, send smarter, and constantly monitor.

Tom is absolutely correct, warming up new IP addresses requires research, preparation and diligence.

The good news for Message Systems customers is that we’ve taken care of this for you with our Adaptive Delivery module. Adaptive Delivery will automatically identify new IP addresses, set initial throttles and gradually increase volume as the IPs age, monitoring ISP responses to ensure that the ISPs are responding positively. If at any point the Adaptive Delivery module identifies a negative ISP response, it adjusts throttles in realtime and monitors for additional negative responses. If an ISP replies with a deferral response, Adaptive Delivery will suspend delivery, throttle back and send you an alert so that you can check the content being sent. All of this is built using intelligence that is constant reviewed and improved by a full-time, in-house deliverability specialist. In addition, our bounce processing system now supports live updates, allowing us to improve classifications thanks to automated feedback from customer systems. If you’re not taking advantage of these new capabilities contact Message Systems and we’ll help.