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5.24. What in the world is Throttling?
Updated 01.01.18
So, you sent an email, and when you checked your Track Results, you found all the messages hadn't been delivered within 5 minutes, or you saw a number of messages marked as Bounced with a status of Temporarily Throttled. What? Why aren't they all delivered yet?
First of all, rest assured that our system is designed with throttling (also known as "deferral" - these terms mean the same thing in the email world, and may be used interchangeably) in mind. We continue to attempt to send messages for up to 72 hours until the message is delivered. Normally, it takes between a few minutes to a few hours until the message is delivered.
But what is throttling? Well, it's when a receiving server refuses a message, but with a specific explanation that tells our server it's only a temporary refusal. They usually say things like "user (or server) is receiving too much mail right now, please try again later", or "user's mailbox is over quota." More info.
What can cause a deferral?
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The receiving server is afraid you're sending spam.
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The receiving server doesn't have any open ports to receive email.
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The recipient's mailbox is full.
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Some recipients have marked your mail as spam, but not enough for the server to block you. Instead, they will refuse to receive more until they see what the rest of your current recipients do.
What happens if it's deferred for 72 hours? That generally means it's an issue like the account being inactive, having a full mailbox, or some other legitimate problem with the recipient. When we are unable to deliver a message after this time period, we convert these deferrals to "Hard Bounces". When an address is hard bounced either due to this, or because we got a different class of error (such as "this user does not exist"), we add this address to the Block list.
Once an email address moves to the Block list, even if you send a message through us for that user, we will not try to deliver to that address, because we know it's no longer good. Continuing to try to send to a known bad address will harm your reputation with the receiver, so we prevent that. However, if you continue to try to send, and we intercept it, your reputation with us goes down (yep, we keep track of that). That could have a negative affect on delivery of future messages you send through us.
Recap
Deferrals, aka Throttling, aka Soft Bounces are a normal part of email delivery, one we know how to handle. There are a number of mundane reasons that can cause a defer. If a message stays throttled, it will be added to the Block list.