16 july 2024, 08:08
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Throttling is an issue many email marketers face. For all we know, email providers could be throttling your emails currently even without your knowledge. What is the cause of email throttling, and what should you know about it?
What is email throttling?
Email throttling occurs when email providers intentionally limit the number of emails they allow from your server during a specific period. If you try to send more emails than the set threshold, the receiving server will "throttle" them, i.e., temporarily reject or put them on hold.
When a mailbox provider throttles your emails, you start encountering “soft bounces” with a message similar to "User's mailbox is over quota. Please try again later." This situation can spook email marketers, but luckily it isn't fatal and can easily be resolved.
What factors cause email throttling?
Mailbox providers may throttle emails for various reasons, such as:
- You are sending too many messages
- The receiving server doesn't have sufficient storage space for your emails
- The receiving server regards your emails as spam-like
- You were previously reported for spam, or your domain is found on a blacklist
The most common reason is possible spam, as mail providers are wary of unknown servers sending lots of emails. However, your throttled emails may not be blocked entirely: the mail system will observe how existing recipients react and decide if it’ll let the throttled emails in on subsequent attempts. But if recipients report the email as spam, the remaining copies will be permanently rejected.
Why is regulating email sending volume important?
Spam is the core reason for mailbox providers to regulate incoming email traffic. They face mammoth spam volumes; for reference, spam accounts for 47% of email traffic. Thus, the work needed to counter it is enormous.
Sending a large number of emails within short time frames is a typical spammer activity pattern. Mail systems take extra caution when they observe such activity, especially coming from IP addresses they aren’t already familiar with. Their modus operandi is to throttle the emails and inspect the ones already received. If recipients report them as spam, the service blocks the sender’s IP address from sending further messages. If there are very little or no spam reports, the ISP gradually lets the throttled emails pass on to their intended recipients. That’s to say, you have nothing to worry about if you are not a spammer.
Email service providers may throttle emails on their end (before they are sent) as a security measure to prevent their IP addresses from being blacklisted for spam. This usually occurs as part of IP warmup for new clients.
How to prevent email throttling
1. Perform IP warmup
The term IP warmup stands for self-regulating the pace of your email sending. After registering a new IP address for your campaigns, you start by sending a few hundred emails per day and gradually increase the volume as time goes on. The goal is to build a solid IP reputation so that your messages are delivered without hassles.
As reputation services and mail providers observe that your IP address sends legitimate messages, they’ll easily allow higher volumes of messages. In contrast, if you send a large volume of messages immediately after registering a new IP, mailbox providers will get suspicious and throttle the messages for further investigation.
The good news is that many email service providers, e.g., UniOne, implement automatic warmup procedures without the need for external tools. This feature helps you avoid throttling and eliminates the need for self-regulating the volume of your emails.
2. Clean up your mailing list
You should frequently scour your mailing list to remove inactive and invalid addresses. This practice, called email list hygiene, also helps prevent email throttling.
Sending emails to invalid addresses increases your bounce rate. A high bounce rate causes mail systems to be wary of further emails from your IP address because it feels like your lists were harvested. Regular list cleaning means you’ll only send emails to active addresses, reducing your bounce rate and improving your chances of avoiding email throttling.
3. Use traffic categorization
It's advisable to separate the traffic for your marketing and transactional emails, which means maintaining separate IP addresses for each type. This tactic enables you to build reputation scores for each one separately and avoid issues with one side interfering with the other. For instance, if your marketing emails are getting throttled, you can still send transactional emails containing time-sensitive information. Otherwise, your critical transactional emails could be delayed, and this would frustrate your customers.
Email throttling best practices
1. Use an email service provider (ESP) for email deliveries
Choosing a reliable email service provider (ESP) is the primary factor for avoiding email throttling issues. Reputable ESPs use IP addresses with a solid reputation, which means you’re less likely to see your emails throttled.
ESPs also regulate email delivery speed so that your messages aren’t throttled by the receiving systems. Each ESP has its limits, but these are reasonable limits that comply with existing restrictions set by major mailbox providers. Finally, most ESPs will automatically re-send any occurring soft bounces, eliminating the need to keep track of your throttled emails manually.
2. Adjust your speed
Some ESPs let you specify a custom sending limit. In that case, 500 to 10,000 emails per hour is a reasonable number. Start with 500 if you’re using a newly purchased dedicated IP address, and gradually increase it as you build a positive IP reputation. On a shared IP, 10,000 emails per hour is a safe sending limit. For anything above this level, contact your ESP for any additional requirements and restrictions.
3. Organize your mailing lists
Organize your mailing list, ensuring invalid, inactive, and duplicate email addresses are scrubbed off. Use free tools like Verifalia and ZeroBounce to check for invalid addresses and remove them. Remove duplicate addresses to avoid sending the same message multiple times to one recipient. Segment large lists for better focusing and engagement. These steps go a long way in preventing email throttling issues.
Final Words
ISP throttling often poses a problem to email marketers, but it can easily be managed and resolved. We’ve explained email throttling, its causal factors, and the best practices for preventing and resolving it. Follow these tips, and you’ll hardly ever face throttling issues.
Above all, choose a reliable and secure email service provider (ESP) like UniOne. Reputable ESPs offer secure, well-maintained servers to deliver emails and manage email-sending speeds to avoid throttling.
