We discussed email throttling in a previous post called Email Throttling Basics, but we wanted to revisit the subject and talk a little more about how throttling works within the SendGrid platform and how it affects email sending.

Email Throttling Causes

Some ISPs limit (throttle) the amount of email they accept from a particular sender during a specified period of time. If you try to send email above their acceptable threshold, they will reject your email resulting in a high number of bouncebacks. This is email throttling and is also referred to as a “deferral.”

This refusal to deliver your mail is usually temporary, but it depends on the situation. When your emails are throttled, you will get a message that says something like “user is receiving mail at too great a rate right now, please try again later,” or “user’s mailbox is over quota.” This is almost always a 400 class of error and can be cleared up within 72 hours.

When an ISP refuses to deliver your mail, it’s usually for one of the following reasons:

  • The receiving server doesn’t recognize your IP address or addresses and thinks you’re sending spam. This is why warming up is so important, so they know who you are. (More on this below, or check out our blog post on the topic.)
  • The receiving server doesn’t have any open ports to receive email.
  • The recipient’s mailbox if full.

Some recipients have marked your email as spam, but not enough for the server to block you. Instead, they will refuse to receive more until they see how the rest of your recipients respond. It’s important to be aware of email throttling as it can have a direct effect on your sending reputation as well.

How SendGrid handles email throttling

Whether you are using SendGrid for email marketing or transactional purposes, we will continue to try and send your email for 72 hours. If the email is still undeliverable after this time period, SendGrid will treat these deferrals as “soft bounces.” We record the soft bounce, but we will not suppress that address if you mail it again. When this deferral happens, there is usually a legitimate reason for the delivery failure—like a full mailbox or inactive account, so you should evaluate your list to make sure you have good emails on your list.

However, this could also mean that your IP doesn’t have a solid reputation—at least not yet.

If you are sending mail over a new IP, then you will need to “warm it up” by sending small amounts of email over a period of time (usually 30 days) in order to build a reputation and introduce yourself to the ISPs.

By gradually increasing the volume you send out, you’ll be able to notice trends in your lists. This approach can be less of a headache than if you send to your entire list in one go. Warming up can be done aggressively or more leniently—it largely depends on if recipient ISPs are accepting your mail. If you pay attention to when a recipient ISP is throttling your mail (using our Event Webhook), you will know when you need to pull back from sending to certain domains.

Email Throttling Prevention Strategies

If you don’t have the benefit of a customer service team like ours, here are a few things you can do to get around the throttling issue:

  • Schedule your emails to deploy over an extended period of time.
  • Segment your emails by domain or split your lists into multiple parts–if you want to start off on the right foot, consider separating your marketing and transactional email traffic to keep their reputations independent.
  • Send your emails at earlier times so that all emails can be sent by your “completions” date.

Email throttling is a normal part of email delivery and each ISP has their own threshold making it hard for you to determine how to deploy your emails without the help of an experienced email deliverability expert. However, if you follow best practices, soon you won’t have to worry too much about email throttling. ISPs will get to know you as a legitimate sender and deliver your emails without fail.

To learn more about email best practices, download SendGrid’s Ultimate Email Deliverability Guide.


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.