Receiving mailboxes (ISPs) often have a limit on how much mail they can receive during a specific timeframe. If you’re sending them too much mail, or sending it too fast, they can choose to defer mail in a process known as throttling.
Reasons for throttling:
- the recipient’s mailbox is full
- your reputation is still building up, so ISPs may fear you’re spamming them
- the receiving server is having bandwidth issues or ran out of open ports
How can you address throttling? The best way is to schedule your emails to go out gradually, and most email service providers (ESPs) do this for you. You can also split up your marketing and transactional mail to use separate sending infrastructures so one’s reputation doesn’t impact the other.