Deliverability
Mailbox rotation planner
Split today's send evenly across an existing mailbox pool, and flag any mailbox pushed past the safe figure you set.
Free, no accountIn your browser
How many messages you plan to send today, across the whole pool.
How many mailboxes exist today. The sending capacity calculator is for deciding how many that should be.
Your own figure, not an official one: no provider publishes a per-mailbox daily limit.
Result
Enter today's volume and the pool size to split the batch
This planner has nothing to split yet: today’s send volume and how many mailboxes exist to send it from.
What this is
This mailbox rotation planner takes a pool that already exists and one day’s volume, and splits that volume evenly across it - with a remainder that cannot divide evenly going to as few mailboxes as possible, which is the part a simple average hides. It flags a mailbox only against the safe daily figure you supply, never against a number this tool invented.
A mailbox pool is only as useful as what gets sent through it evenly. A batch that divides cleanly looks fine on average, but if it does not divide cleanly, a handful of mailboxes quietly carry one more message than the rest - and if that extra message tips them over a safe limit, the average told you nothing.
This is a scheduling question, not a sizing one: the sending capacity calculator decides how many mailboxes and domains a target volume needs in the first place. This planner assumes that decision is already made and asks a narrower question - given what exists today, where does today’s batch actually land?
How to read the result
The split is floor division with a remainder: today’s volume divided by the mailbox count gives a base amount every mailbox gets, and whatever is left over after that division goes one at a time to as many mailboxes as there are left, so those mailboxes carry one extra message each rather than the remainder being invented or dropped.
A mailbox is flagged only once its figure is compared against the safe per-mailbox volume you typed in. Leave that field at zero and the split is still shown, but nothing is marked over or under, because there is nothing supplied to compare it against.
Common mistakes
What goes wrong most often, and how to tell.
- Reading the average as the figure to check against a safe limitAn average of, say, 38 sends a mailbox can hide a handful of mailboxes actually carrying 40 because the batch did not divide evenly. It is the per-mailbox figure after the remainder lands that matters, not the mean.
- Rotating the same mailboxes into the "extra message" group every dayThis planner says how many mailboxes need to carry one more message today, not which ones. Picking the same few every day defeats the point of spreading load, by concentrating the extra volume on the same inboxes instead of rotating it.
- Confusing this with the tool that decides how big the pool should beThis planner cannot grow the pool for you - if every mailbox is over your safe figure even on the even share, the honest fix is more mailboxes or less volume today, not a looser reading of the split.
Questions
What people ask about this tool.
- Divide today’s total by the number of mailboxes: the whole part is what every mailbox gets, and whatever is left over goes one at a time to as many mailboxes as the remainder, so those few carry one extra message rather than the leftover being guessed at or ignored.
- The capacity calculator sizes the pool: it answers how many mailboxes and domains a target volume needs before any of them exist. This rotation planner assumes the pool already exists and answers a narrower, daily question - given what you have, where does today’s batch land, and is any mailbox pushed past the safe figure you set?
- No mailbox provider publishes an official daily limit for one mailbox, so there is no number this tool could assume without presenting a guess as a rule. The figure you type is the only one it checks the split against.
- A handful of mailboxes - exactly as many as the remainder - send one more message than the rest. The table shows both groups and how many mailboxes are in each, so it is visible which share of the pool is carrying the extra load today.
Use it from the API
The same check, as one request.