Deliverability
Mailbox calculator for cold email
How many mailboxes and domains a daily send target needs, once you say what one mailbox can safely carry.
Free, no accountIn your browser
How many emails a day you want this pool able to send, across every mailbox together.
Your own figure, not an official one: no mailbox provider publishes a per-mailbox daily limit. Cold-email guides commonly cite 20-50 a day as ESP practice, not a vendor rule.
A planning choice, not a published rule either. Three is a common starting point.
Result
Enter a target volume and a safe per-mailbox figure to size the pool
This calculator has nothing to divide yet: a target daily volume and the daily volume you consider safe for one mailbox.
What this is
This mailbox calculator cold email senders rely on turns one target daily volume into the mailbox and domain count a pool needs, once you supply the one figure nobody else will give you: a safe daily send rate for a single mailbox. No mailbox provider documents that number, so it is asked for rather than assumed, and the mailboxes and domains come out of your own figure rather than a hidden default.
Buying mailboxes and domains for cold outreach is the single most direct next step this whole catalogue of tools leads to, which is also why getting the input wrong here is expensive: a pool sized off a made-up per-mailbox figure is either too small to hit the target or too large for the budget it was supposed to fit.
The arithmetic is two divisions and two round-ups: a target volume divided by a per-mailbox rate gives the mailbox count, and that count divided by how many mailboxes sit on one domain gives the domain count. Neither division assumes anything you did not type.
How to read the result
Mailboxes needed is the target divided by your safe per-mailbox figure, rounded up - a mailbox comes whole, so a fraction still needs a whole extra one. Domains needed takes that mailbox count and divides it again by how many mailboxes you plan to run on one domain.
The reference table below shows what a handful of commonly cited per-mailbox figures from cold-email practice would produce for the same target, with your own figure marked among them. It exists so your number can be seen against others people quote, not because any row in it is this tool’s actual answer.
Common mistakes
What goes wrong most often, and how to tell.
- Treating 20-50 a day as an official limitThat range comes from ESP practice and cold-email guides repeating each other, not from a page Google or Microsoft publishes. Using it without adjusting for a mailbox’s own age and warm-up state treats a borrowed guess as a vendor rule.
- Sizing the pool once and never revisiting itA safe per-mailbox figure early in a mailbox’s life is lower than it is once it is fully warmed up. A pool sized for day one stays undersized for the volume the same mailboxes could carry a month later.
- Ignoring the domain count because mailboxes feel like the real costEach domain carries its own reputation separate from the mailboxes on it. A pool with too few domains concentrates risk - one domain’s problem affects every mailbox sitting on it - even when the mailbox count alone looks healthy.
Questions
What people ask about this tool.
- Divide your target daily volume by the daily volume you consider safe for one mailbox, then round up. At 500 a day and a self-set limit of 40 a mailbox, that is 12.5, so 13 mailboxes - 12 would fall short of the target every single day.
- Nobody can tell you an official one: no mailbox provider publishes a daily limit for a single mailbox. Cold-email guides commonly cite 20-50 a day as a starting range, but that is shared practice, not a rule, and a mailbox’s own age and warm-up state move it either way.
- Take the mailbox count this calculator gives you and divide it by how many mailboxes you plan to run on one domain. Three mailboxes a domain is a common starting choice, but it is your choice to make, not a published limit.
- Because assuming one would mean quietly presenting someone else’s guess as a rule. The 20-50 range shown for comparison is exactly that - a guess people repeat - and the mailbox and domain counts above are only trustworthy to the extent the figure typed into the form actually is.
Use it from the API
The same check, as one request.