How sending windows and daily limits work in Tantra

Overview

Two things control how fast your cold email goes out: a sending window that defines when sending is allowed, and per-mailbox caps that limit how many emails each mailbox sends per day and per hour. Unlike calendar campaigns, these per-mailbox caps are enforced on every send.

How it works

Before every send, Tantra runs two checks:

Sending window. Is the current time inside the days and hours you allowed? If not, the step waits.
Quota. Does the mailbox have room under its daily and hourly caps? Tantra reserves a slot atomically, so two sends can never both take the last slot.

Only when both pass does the email go out. A mailbox also waits at least half a second between sends, which keeps it under Gmail's per-mailbox rate.

The sending window

Set the window in Step 5 of the builder.

Timezone. The window is measured in this timezone. If a contact has their own timezone on record, Tantra uses the contact's timezone for that recipient instead.
Days of the week. Toggle the days sending is allowed. At least one day is required.
Start hour and End hour. The hours sending is allowed, on the hour. The start hour must be before the end hour.

Outside the window, due steps do not send. They wait and fire when the window next opens. This is true even if you chose Start now.

Sending window controls

Per-mailbox caps

Each mailbox has a daily cap and an hourly cap. These are the limits Tantra enforces at send time.

Default daily cap: 50 emails per mailbox per day.
Default hourly cap: 25 emails per mailbox per hour.

You set these per mailbox on the Accounts page, using the email cap / day and email cap / hour inputs on each connected mailbox. Editing them there changes the caps that are actually enforced.

Google also applies a Workspace ceiling of 1500 recipients per mailbox per day. A mailbox cap only ever lowers your limit below that ceiling. It never raises it.

The per-mailbox cap fields in the builder

Step 5 also has Daily cap (per mailbox) and Hourly cap (per mailbox) fields, with tooltips saying they override each Account's limit for this campaign only. The allowed ranges are 1 to 2000 for daily and 1 to 500 for hourly. These values are saved on the campaign.

Unable to verify from the available source code. that these campaign-level fields override the per-mailbox caps at send time. The caps confirmed to be enforced are the per-mailbox caps set on the Accounts page. To be sure a limit takes effect, set it on the mailbox in the Accounts page.

Sender rotation

When you select several mailboxes, Sender rotation decides how enrollments are spread across them:

Round-robin. Spread evenly across mailboxes.
Fill first. Exhaust one mailbox before using the next.
Random. Assign at random.

A contact's mailbox is chosen when they enroll and stays the same for every step, so the whole conversation comes from one address.

Field reference

Control	Where	Rules
Timezone	Step 5 sending window	Any timezone. Contact timezone overrides per recipient.
Days of the week	Step 5 sending window	At least one day.
Start hour / End hour	Step 5 sending window	On the hour. Start before end.
Daily cap (per mailbox)	Step 5	1 to 2000. Stored on the campaign.
Hourly cap (per mailbox)	Step 5	1 to 500. Stored on the campaign.
email cap / day	Accounts page, per mailbox	Enforced daily cap. Default 50.
email cap / hour	Accounts page, per mailbox	Enforced hourly cap. Default 25.
Sender rotation	Step 5	Round-robin, fill first, or random.

What happens next

When a step becomes due:

Inside the window, with quota available: the step sends within about a minute.
Outside the window: the step waits and is retried when the window next opens.
Hourly cap reached: the send is deferred and retried later in the day, after the hour rolls over.
Daily cap reached: the send is deferred to the next day, around 09:00.

None of these are errors. They are the throttle working. You can watch each contact's next send state on the campaign's enrollments view.

Best practices

Keep caps conservative for new mailboxes and raise them slowly. See Warm up your sending mailboxes.
Match your window to business hours in your audience's timezone.
Add more mailboxes to raise total volume instead of pushing one mailbox to its ceiling.

Common mistakes

Blaming a stalled send on a bug. Outside the window or at a cap, waiting is expected.
Setting a limit only in the builder. Set enforced caps on the mailbox in the Accounts page.
Expecting to beat Google's ceiling. A mailbox cannot send more than 1500 recipients per day regardless of your settings.

Troubleshooting

Symptom	Cause	Fix
Campaign is active but no emails have sent	Current time is outside your sending window.	Wait for the window, or widen the days and hours.
Sending slowed to a trickle	A mailbox hit its hourly cap.	Sends resume after the hour rolls over, or raise the hourly cap on the mailbox.
Sending stopped for the day	A mailbox hit its daily cap.	Sends resume the next day, or add more mailboxes.
A mailbox cap I set in the builder seems ignored	The enforced cap is the one on the Accounts page.	Set the cap on the mailbox in the Accounts page.
No emails at all from one sequence	No connected sender, or all outside window.	See My emails are not sending.

FAQ

What are the default per-mailbox limits? 50 per day and 25 per hour, per mailbox.

Can I send more than 1500 a day from one mailbox? No. That is Google's Workspace ceiling. Mailbox caps can only lower your limit.

Does the sending window apply to Start now campaigns? Yes. Even with Start now, sends wait until the window is open.

Which timezone is used for the window? The window timezone you set, unless a contact has their own timezone, in which case that contact's timezone is used.

Where do I change the enforced caps? On the Accounts page, per mailbox, using the daily and hourly email cap inputs.

Do email caps and calendar invite limits share the same budget? No. They are tracked separately, so cold email and calendar invites do not consume each other's allowance.

Sending windows and daily limits