What is the time-to-deliver metric?
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Time-to-deliver (TTD) is how long it takes for an email to reach the recipient's mail server after you hit send. It starts when your ESP accepts the message and ends when the receiving server accepts it. (That's different from when the person actually opens it.)
TTD includes everything that happens in between: queue time at your ESP, network latency, retry attempts if the receiving server is busy or throttling, and greylisting delays if the mailbox provider wants to verify you're legitimate. A normal TTD is under a minute for transactional email, but can be several minutes (or longer) if the receiving server is cautious or your sending reputation is still warming up.
Why TTD matters: if you're sending password resets or order confirmations, a 10-minute delay feels broken to the user. For marketing emails, TTD mostly matters at scale. If your ESP is taking 30 minutes to clear your campaign queue, that's a sign of either volume throttling or infrastructure issues on your end.
Most ESPs show TTD in their logs or analytics dashboards. Postmark reports it to the second. SendGrid and Mailgun show delivery timestamps you can compare to send timestamps. If your ESP doesn't surface this metric, you can estimate it by comparing your send logs to the receiving server's Received: headers (check our free Email Header Analyzer to parse them).
Common causes of high TTD: greylisting (the receiving server forces a retry, usually 5-15 minutes), rate limiting (you're sending too fast for the recipient's server to accept), your sending IP has reputation issues so the server is being extra cautious, or your ESP's infrastructure is overloaded. If TTD suddenly spikes across all mailbox providers, that's your ESP's problem. If it spikes only at one provider (say, all Gmail delays but Outlook is fine), that's a reputation or authentication issue.
But You can't control greylisting or recipient-side throttling, but you can avoid TTD problems by warming up new IPs properly, keeping your bounce rate low, and making sure your SPF and DKIM are set up correctly. If you're seeing consistent 10+ minute delays and you're not a brand-new sender, that's worth investigating with your ESP or asking us (free help at reviewmyemails.com/sos).
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