Mailbox sending capacity

Mailbox sending capacity is a planning problem with separate ceilings: the provider's technical limit and the volume your domain can carry without reputation damage. Start by separating the question "How many emails can I send per day?" from "How many can I send without burning my domain?"¹ The capacity question points to service limits; the reputation question sets the operating budget. A provider may accept traffic that still damages the domain, so plan from the lower practical limit.² When several inboxes share a domain, the practical limit applies across the domain, so splitting volume between inboxes does not create fresh capacity.³

Run the capacity check

1

Provider boundary

2

Domain state

3

Shared budget

4

Mailbox plan

5

Review

1. Provider boundary
2. Domain state
3. Shared budget
4. Mailbox plan
5. Review

Work through the limits in order. Set the shared domain budget before you decide how many inboxes to provision, then increase volume only after the review signals support it.

Stage	What you are trying to learn	Example question
Provider boundary	Which service limit can stop delivery?	What limit applies to this sender?
Domain state	How much volume the domain can carry now	What is the domain's current sending stage?
Shared budget	What total volume can run across the domain?	What daily volume can all inboxes share?
Mailbox plan	How many inboxes the campaign needs	How many contacts enter the sequence during the sending window?
Review	Whether volume can increase	What changed in engagement after the last increase?

Set the provider boundary

Map the limit's unit before you plan daily cold email volume. A minute limit, an external recipient limit, and a daily recipient limit can govern different parts of the same sending system.

Azure Communication Services lists custom-domain Send Email at 30 emails per subscription per minute, with a higher limit available.⁴ The same service lists 100 emails per subscription per 60 minutes, again with a higher limit available.⁵ Treat those as service boundaries that need to be checked against your actual sending pattern.

The external message limit covers the maximum number of external recipients per hour.⁶ The daily message limit covers the maximum total number of recipients per day.⁷ Write down which control governs your sender, then request a higher sending limit before reaching the current one when the plan requires it.⁸

Move on when you know the provider limit, the time window it uses, and whether it counts messages or recipients.

Set the practical budget

The provider boundary tells you what the system can process. Your domain stage sets the operating budget for cold outreach.

For a new domain in weeks 1 to 2, send 10 to 30 emails per day.⁹ During weeks 3 to 6, a warming domain can send 30 to 75 emails per day, with increases conditional on healthy engagement.¹⁰ An established domain with a solid reputation is limited to 75 to 150 emails per day, which is a ceiling for most cold outreach.¹¹

Apply that schedule to the domain's combined traffic. Then allocate the budget across inboxes. At the mailbox level, the recommended range is 20 to 30 cold emails per inbox per day, with 50 as the hard ceiling.¹² If the domain budget allows less than the mailbox plan, use the lower domain budget.

Move on when the sum of mailbox allocations fits inside the shared domain budget.

Size the infrastructure

Turn campaign demand into mailbox demand before assigning contacts. This keeps the sending plan tied to the sequence and its window instead of a number chosen for convenience.

Estimate mailbox demand by multiplying sequence length by the contacts entering it, dividing by the sending window in days, then dividing by 20 to 40 sends per mailbox per day.¹³ Use the lower end of that planning range when you need more margin. Check the result against the per mailbox and domain budgets before provisioning.

Increase volume through more inboxes across more domains, keep each inbox at a volume that looks human, and ramp each one on a schedule.¹⁴ A campaign that needs more capacity should add sending infrastructure while preserving the mailbox budget. If the infrastructure calculation exceeds the current domain budget, extend the sending window or add domain capacity.

Do not let the mailbox calculation hide the domain constraint. A group of inboxes can fit their individual allocations and still exceed what the shared domain should carry.

Monitor and adjust

Engagement

If engagement remains healthy

Continue the planned ramp

If it does not

Hold the volume

Engagement

• If engagement remains healthy, continue the planned ramp
• If it does not, hold the volume

Capacity planning ends with a review loop. Record the volume and the response after each change so an increase has a reason behind it.

Monitor overall email use, per content use, and per recipient use.¹⁵ Review those views before increasing volume, and keep the sending limit visible at both mailbox and domain level.¹⁶ If engagement remains healthy, continue the planned ramp; if it does not, hold the volume and investigate before adding more sends.

Keep the operating record simple: provider boundary, domain stage, combined domain volume, mailbox allocation, and the engagement result after a change. That record tells you whether the limit came from the service, the domain, or the way the campaign was distributed.

What not to do

Avoid the shortcuts that turn technical capacity into reputation damage.

• Do not turn the provider's sending ceiling into a planned daily budget.¹⁷
• Do not push a freshly warmed domain past 50 emails per mailbox per day, because that can trigger ESP rate limiting and spam classification.¹⁸
• Do not let a mailbox plan replace monitoring of overall, per content, and per recipient use.¹⁵

Use the provider boundary to rule out impossible schedules, the domain stage to set the practical budget, and the mailbox calculation to decide how much infrastructure to provision. Increase capacity through paced infrastructure only after the review supports the change.