Some internet service providers (ISPs) limit—or throttle—the amount of email accepted from a particular sender during a specific period. If you try to send an email above the acceptable threshold, the ISP will reject your email, resulting in higher bounces back. 

When ISPs throttle your emails, you’ll get a message that says something like, “User’s mailbox is over quota” or “User is receiving mail at too great a rate right now, please try again later.” A temporarily blocked delivery attempt that asks you to send the email later is a type of email throttling called a deferral. 

As a 400 Bad Request error code, email throttling can be a thorn in an email marketer’s side, but fortunately, you can resolve most issues within 72 hours. Depending on your situation, you might even find there’s a silver lining to certain types of email throttling. 

Is email throttling bad?

Email throttling gets a bad rap, but in some cases, spreading out email delivery has its benefits:

  • For one, you can regulate the number of emails delivered during high-traffic periods. This move can maximize response to your email campaigns and provide fertile testing ground for future sends. 
  • Intentional throttling can also help you manage responses. For instance, if your call to action is a phone call, staggering your email communication yields more manageable call center volume that allows you to operate with a leaner customer service staff. 
  • Likewise, you can manage server bandwidth to your website better when marketing special promotions and sales.

However, if you are a high-volume sender whose business model relies on timely delivery to reap the maximum response, throttling (or rate limiting) can prove to be a hindrance. 

Daily deal sites are a great example. These have a short window of opportunity to achieve sales and, therefore, need all email messages to reach subscribers immediately. In this case, your email reputation plays a crucial part in determining delivery. 

All that to say, it’s critical to look deeper into what causes email throttling and address these issues, as email throttling causes delivery failure, affects your sending reputation, and prevents the deliverability of future messages as well.

What causes email throttling?

An ISP typically refuses to deliver email for one of the following reasons:

  • The recipient’s email mailbox is full
  • The receiving server doesn’t have any open ports to receive email
  • The receiving server doesn’t recognize your IP address(es) or thinks you’ve sent spam

In other words, the ISP will typically refuse to deliver to recipients when some recipients mark your email as spam but not enough for the server to block you permanently. Instead, it’ll refuse to receive more messages until it sees how the rest of your recipients respond. Either way, it’s bad news. You want your messages to reach recipients on time and ISPs to consider you a trustworthy sender of quality emails. 

If ISPs throttled you in the past, it probably stings, but Twilio SendGrid has back-end analytical tools to provide you with increased visibility into what’s happening and why.

How Twilio SendGrid handles email throttling

As a cloud-based SMTP provider that allows you to send email without having to maintain servers, SendGrid has unique insight into the world of email throttling and a few recommendations on what you might do when it happens.

  • Do nothing. Whether using SendGrid for email marketing or transactional purposes, we’ll continue to try and send your email for 72 hours. 
  • Try again. If the email is still undeliverable after the 72-hour period, SendGrid will treat these deferrals as soft bounces. And while we’ll record the soft bounce, we won’t suppress that address if you send email to it again. 
  • Scrub your contact list. When a deferral happens, there’s usually a legitimate reason for the delivery failure—like a full mailbox or inactive account—so evaluate your contact list to ensure you have correct, complete, and current emails in your database. Follow these 5 email list hygiene tips for the best results. 
  • Build your reputation. A deferral could also mean that your IP doesn’t have a solid reputation—at least not yet. If you’re sending mail over a new IP, you’ll need to warm it up by sending small numbers of emails over a period (usually 30 days) to introduce yourself to the ISPs and develop a favorable reputation. If you’d like to learn more on this topic, check out SendGrid’s Email Guide for IP Warm Up.
  • Start small, look for patterns, and time it right. By gradually increasing the volume you send out, you’ll notice trends in your lists. Starting small is less of a headache than if you send to your entire list in one go, but pay attention to when a recipient ISP throttles your mail (using our Event Webhook). You’ll know when you need to pull back from sending to certain domains.

Since ISPs impose strict standards on suspected spammers, working with an experienced email provider can make a big difference in how ISPs treat your messages—that’s where we come in. When you partner with the Email Experts at SendGrid, we help you navigate email throttling issues long before your sending reputation is at stake. Contact us to learn more

How to prevent email throttling with Twilio SendGrid

Want to know how to prevent email throttling? Here are a few things you can do: 

  • Schedule emails to deploy over an extended period
  • Segment emails by domain or split your lists into multiple parts
  • Separate marketing and transactional email traffic to keep reputations independent
  • Send emails at earlier times to deploy by your completion date

Email throttling is just part of email delivery, and each ISP has its threshold, which makes it a challenge to determine how to deploy your emails without the help of an email deliverability expert such as Twilio SendGrid. 

However, if you follow the best practices found in our Email Deliverability 101 and Ultimate Email Deliverability Guide, email throttling will soon be a concern of the past. ISPs will get to know you as a legitimate sender and deliver your emails without fail.



Author
As Twilio SendGrid's Sr. Director of Content, Jillian is responsible for ensuring that SendGrid provides valuable thought leadership content through the blog, whitepapers, webcasts, and more. When not writing and editing, you can find Jillian frequenting Denver restaurants in search of the best queso.
Ayanna Julien
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As Twilio SendGrid's Editorial Marketing Manager, Ayanna owns the quality assurance of blog content by focusing on authentic storytelling and editorial integrity. When not editing, Ayanna enjoys reading a good fiction novel or writing her own fiction.