So, you tried to send a bunch of emails, and when you checked your Event Webhook or your Email Activity page, you see a number of messages are marked as "Deferred" instead of "Delivered". Or maybe you're noticing a lot of addresses in your Blocks, but not many in your Bounces. What? Why aren't your recipients currently in awe at the glory of your amazing message?
SendGrid continues to attempt to send messages for up to 72 hours or until successful delivery. Normally, it takes between a few minutes and a few hours for a deferred messages successful delivery - it depends on the receiving mail server.
Throttling is when a receiving server refuses a message, but with a specific explanation that tells our server that it's only a temporary refusal. The error is usually something like "user is receiving mail at too great a rate right now, please try again later", or "user's mailbox is over quota." These errors are most often 400 errors - indicating a temporary failure. They are not often 500 errors - indicating a permanent failure.
If a message is continuously deferred for 72 hours, it may mean that there is another issue such as the account being inactive, 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 "Blocks". Blocks are stored for reference and do not behave like a suppression list when sending to addresses that are on the list.