The ability to throttle message delivery can often be an overlooked concept due to all the other elements of deliverability. Simply adjusting the time that your message is sent over can have a huge impact, particularly when mailing to certain domains and domain families that may be blocking a portion of your messages. Throttling can also serve to manage responses to your messages, if for example, you’re encouraging your recipients to dial a number.
Signs that you may need to throttle messages can appear when you break down your delivery by domain or domain family. Certain ISPs will give you very detailed bounce codes that may be related to sending emails too quickly, like the RP-001 error from Outlook.com, and the TS01 and TS02 bounce codes from Yahoo. These are definite signs that your mail is being sent too quickly to these recipients, and experimenting with your message throttling settings would be very helpful. Beyond these obvious signs, you can also adjust your throttling time due to an increase in bounce rates, spam complaints, spam trap hits, or a loss of subscriber engagement. All of those negative indicators can be warning signs that your messages are being sent too quickly!
Message throttling is also very important when warming up dedicated IPs, as ISPs are more wary of your sending until they have an idea of the type and volume of messages being mailed. Until an ISP has enough data to know your IP reputation, you’ll be subject to increased scrutiny and rate limits. Throttling in these cases also carries some hidden benefits, like lessening the impact of complaints. An ISP that sees a sender mail 10000 messages in a single hour and then sees 15 spam complaints is certain to harm your reputation more than those complaints spread out over 8 hours. This will help keep your reputation clean, and make warm up a much smoother process. A similar side effect can be being blacklisted due to sending too quickly, which can be a very painful and time consuming process to clear yourself from. Several ISPs have policies where they mention that the volume of email they can process per hour is a direct function of the reputation score of the originating IP address. So sending too quickly can decrease your reputation, which results in fewer emails being allowed per day/hour!
Obviously there are cases where your message needs to be delivered quickly, say with a time specific offer or something similar. Sending without any throttling can be done, but it should only be attempted after you’ve established a good reputation and haven’t seen any bounces related to rate limiting. Since you’re sending to thousands of different domains, it’s impossible to know the rate limit settings of each, and many are changing often. Due to this, experimenting and finding the best throttling time is very important.