start_warmup
Start the ramp on an older mailbox still pending. New senders start warming on their own.
›Start gradual warmup on every mailbox that is still pending.
What it does
Since October 2026 warmup starts by itself on every new sender and is included in the plan, so this is only for a mailbox added before then that is still `pending`.
Moves a mailbox from `pending` into `warming` and puts it on the schedule that raises its daily allowance over time. A warmup schedule maps day ranges to daily limits, so early days stay deliberately small and the ceiling lifts as history accumulates.
Mailboxes graduate `pending` → `warming` → `ready`. Only a ready mailbox is fully eligible for campaign volume, and the progression is driven by the hourly pipeline rather than by you remembering to raise a number.
The ramp is the cheapest deliverability insurance available, which is why nobody has to remember to start it any more.
Starting warmup on several mailboxes at once is fine and usually correct, since capacity comes from breadth rather than from pushing any single address harder.
Parameters
| Name | Type | Notes |
|---|---|---|
| mailbox_id * | string |
* required · Maps to POST /api/warmup/start
What comes back
The updated mailbox with warmup stage `warming`.
The thing people get wrong
A campaign can send from a warming sender, but only up to its warmup limit for the day, and warmup mail counts toward that limit.
Use it with
More warmup tools
See all 147 tools and the connection string on the MCP server page.