Gradually ramp up your sending volume to build IP reputation and avoid throttling by mailbox providers.
Mailbox providers such as Gmail and Outlook evaluate the reputation of the IP address that sends email to their users. A new IP with no sending history is inherently untrusted — providers throttle or reject mail from unknown sources until a consistent, positive sending pattern is established. Warming up means starting at a low volume and increasing steadily, giving providers time to recognize your IP as a legitimate sender.
Each mailbox provider applies its own throttling rules across multiple dimensions, and these rules change frequently. No email service provider (ESP) can fully predict them. Monitor your own delivery signals and adjust volume accordingly.
Step 1: Control your sending frequency
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Set your sending rate using the following formula:
Rate = max(1 email/s, total volume ÷ (8 hours × 3,600 seconds))
This formula spreads your sends evenly across an 8-hour window. Mailbox providers watch the hourly rate — not just the daily total. Sending 50,000 emails in one hour looks far more suspicious than the same 50,000 sent over 8 hours.
If throttling signals appear, extend the window to lower the per-second rate.
Step 2: Find the right starting volume
Set an appropriate initial daily volume before ramping up.
First-time senders: Start at 2,000 emails per day. Double the daily volume each day until throttling occurs.
Senders with existing history: Identify the highest daily volume in your historical data that did not trigger throttling from any mailbox provider, and use that as your starting point.
Step 3: Monitor metrics and scale up
In the Direct Mail console, review the previous day's delivery status. Enable Data Tracking to monitor click and open rates.
Use the following table to decide whether to increase, hold, or roll back your daily volume:
|
Signal |
Action |
|
No throttling errors; delivery metrics are healthy |
Increase daily volume by 5% |
|
Throttling errors detected |
Hold at current volume until errors clear |
|
Open rate declining or bounce rate rising |
Hold and investigate list quality |
Key metrics to watch:
Delivery failures (bounces): Monitor your hard bounce rate. A rising bounce rate indicates list quality issues that must be resolved before scaling.
Open rate: Monitor for trends. A declining open rate suggests recipients do not recognize your sender address, which lowers your reputation score with mailbox providers.
Unsubscribe rate: Watch for spikes, which indicate content or frequency issues.
Spam complaints: Complaint rate is one of the strongest negative signals to mailbox providers. Keep this rate as low as possible.
When scaling without throttling, increase volume by 5% per day. Start conservative and lower the growth rate further if problems appear.