A Claude skill that computes your daily new-lead cap

---
name: inbox-capacity-planner
description: Works out how many new leads a day a set of sending mailboxes can actually sustain, given how long the sequence is, and flags when a planned import will starve its own follow-ups. Use when sizing a mailbox fleet, planning cold email volume, or diagnosing a campaign that looks healthy on the dashboard but has stopped reaching anyone new.
license: MIT
---

# Inbox capacity planner

Most people size a cold email programme by daily send volume. That is the wrong
number, and using it is the most common reason a campaign quietly stops working
in its second week.

Sending capacity is not the rate at which you can add people. Every lead you
add consumes one send for each step of the sequence, spread over the following
days. A five step sequence means each lead costs five sends, not one. The rate
you can sustain is therefore capacity divided by sequence length, and it is a
much smaller number than the one on the invoice for the mailboxes.

This skill computes that number, shows the two separate ceilings that produce
it, and says what will go wrong if the import rate exceeds it.

## What to ask for

Ask for all six. If the person does not know one, say which, and give the
answer as a range rather than quietly guessing a single figure.

1. **Mailboxes**: how many sending addresses are in the fleet.
2. **Sends per mailbox per day**: the configured daily limit per address.
3. **Sequence length**: how many emails one lead receives before the sequence
   ends, counting the first one.
4. **Days between steps**: the gap the sequence waits between emails.
5. **Sending window**: how many minutes a day the fleet is allowed to send in,
   usually working hours in the recipient's timezone.
6. **Minimum gap between sends from one mailbox**: the spacing setting, in
   minutes.

Also ask how large the list is, if there is one. It converts the answer into
days of runway, which is the form most people actually want.

## The arithmetic

Two ceilings apply, and the lower one wins.

The **configured ceiling** is what the sending tool has been set to allow. The
**physical ceiling** is how many sends a mailbox can place inside the window,
given the spacing:

```
slots per mailbox     = floor(sending window minutes / minimum gap minutes)
effective per mailbox = min(sends per mailbox per day, slots per mailbox)
effective daily sends = mailboxes x effective per mailbox

sustainable new leads = floor(effective daily sends / sequence length)
days to steady state  = (sequence length - 1) x days between steps
days of runway        = list size / sustainable new leads
```

**Always report both ceilings, not just the answer.** When the physical ceiling
is the lower of the two, the configured number is fiction: the fleet cannot
place the sends it has been told to place, and the shortfall never appears in a
settings screen. Say so out loud every time it happens.

## The failure mode this is really about

If new leads are added faster than the sustainable rate, the programme does not
fail loudly. It fails in one of two quiet ways, depending on how the sending
queue is ordered.

**If follow-ups go first**, which is the sane default, the daily allowance is
spent on the sequence steps already owed to leads in flight. New leads wait.
Import several times the sustainable rate and the new-lead queue stops draining
altogether: the dashboard reports a full day of sending, every mailbox hit its
limit, and almost nobody new was contacted. Everything looks healthy. This is
the shape of a campaign that appears to send nothing for a day, and the cause
is arithmetic upstream rather than a fault in the tool.

**If new leads go first**, the newest arrivals consume the allowance and the
follow-ups slip. The three day gap becomes five, then nine. The sequence
stretches until the second email no longer reads as a follow-up to the first,
and the reply rate falls for a reason that has nothing to do with the copy
everyone is busy rewriting.

Say which of the two is happening if the person is diagnosing rather than
planning. Ask how their queue is ordered. If they do not know, the symptom
tells you: full send volume with no new contacts is the first, and sequence
gaps drifting longer than configured is the second.

## The levers, in order of cost

1. **Shorten the sequence.** The sustainable rate is inversely proportional to
   sequence length, so going from five steps to four raises it by a quarter,
   immediately and for nothing.
2. **Widen the window or narrow the gap**, but only when the physical ceiling
   is the binding one. If the configured ceiling is lower, this changes nothing.
3. **Add mailboxes.** Linear, and the only lever that costs money every month.
4. **Import more slowly.** Not a lever on capacity, but usually the correct
   answer, and the one nobody suggests.

## Ramping

A domain and mailbox that has not sent to strangers before should not start at
the target volume. During a ramp the sustainable new-lead rate is proportionally
lower, because the capacity it is divided from is lower.

The trap is importing at the steady state rate during week one. That guarantees
a backlog before a single follow-up has been sent, and the backlog is invisible
until the sequence steps come due.

If a ramp is in progress, compute the sustainable rate at the current week's
volume rather than the target, and say which one you used.

## What to output

A short table, then the sentences that matter:

| Figure | What it tells them |
|---|---|
| Effective daily sends | the lower ceiling, and which of the two it was |
| Sustainable new leads per day | the answer |
| Days to steady state | when the full load actually arrives |
| Days of runway | if a list size was given |

Then, in plain sentences: the sustainable rate with the daily send volume next
to it, so the gap between the two is visible; which ceiling is binding and what
would move it; and, if a planned import exceeds the sustainable rate, which
failure mode to expect and roughly when.

Do not present the sustainable rate on its own. The number is unremarkable in
isolation and persuasive next to the number it is usually confused with.

## Worked example

Illustrative figures. These are not measured from any account.

Eight mailboxes, 30 sends per mailbox per day, a five step sequence with three
days between steps, a nine hour window, and a fifteen minute minimum gap.

```
slots per mailbox     = floor(540 / 15) = 36
effective per mailbox = min(30, 36)     = 30      configured ceiling binds
effective daily sends = 8 x 30          = 240
sustainable new leads = floor(240 / 5)  = 48
days to steady state  = (5 - 1) x 3     = 12
runway on 2,000 leads = 2000 / 48       ~ 42 working days
```

The fleet sends 240 emails a day and sustains 48 new people a day. Anyone
planning against the 240 is planning at five times the real rate.

Now change one setting. With a twenty minute gap instead of fifteen:

```
slots per mailbox     = floor(540 / 20) = 27
effective per mailbox = min(30, 27)     = 27      physical ceiling now binds
effective daily sends = 8 x 27          = 216
sustainable new leads = floor(216 / 5)  = 43
```

The tool still says 30 a day. The mailbox can place 27. Nothing reports the
missing three, and the sustainable rate drops by roughly a tenth because of a
spacing setting nobody associates with volume.

## What this deliberately does not do

- It does not know your provider's real limits. Those vary and they change.
  Ask rather than assume, and say in the output that the answer is only as good
  as the limit you were given.
- It does not model deliverability, reply rates or bounce rates. Capacity is
  what you are allowed to send. Whether it lands, and whether anyone answers,
  are different questions needing different evidence.
- It does not tell you which people on the list may lawfully be emailed.
  In the UK that depends on each recipient's legal form, and getting it wrong
  is a legal problem rather than an operational one. Use a compliance check for
  that, before the list is loaded rather than after.

## Not for you if

- You send fewer than a few hundred emails a month. The arithmetic still holds,
  but capacity will not be the thing limiting you.
- You want a deliverability audit. This is a volume model and it will tell you
  nothing about whether your mail reaches an inbox.
- You want one recommended number of mailboxes to buy. The honest answer
  depends on your sequence length and your list size, which is why both are
  inputs rather than assumptions.

---

Written by TheCold.Ai, a B2B outbound agency in Middlesbrough, England.
Not affiliated with Anthropic.