Mailbox sending capacity

Treat mailbox capacity as a reputation budget. Your provider's sending limit tells you when the system will stop you; your operating target marks where deliverability stays healthy. Use the provider limit as a technical ceiling and set a separate safe deliverability target.¹ For planning, use 20 to 30 cold emails per inbox per day and keep 50 as an outer ceiling.² New domains need a slower start, with 10 to 30 emails per day during weeks 1 to 2.³ Plan the infrastructure before the volume.

Set the operating target

Choose the daily target before assigning more mailboxes. Put it beside the domain's warm-up state and campaign volume so the technical maximum does not become the plan by default.

A practical ceiling for a domain is about 10,000 to 15,000 emails per month, depending on mailbox count, warm-up history and overall list quality.⁴ Use that figure to model total domain load, then choose a mailbox pace that keeps the campaign inside the safer range. Ask what daily volume the mailbox can carry while the domain keeps its reputation.

Check provider limits

Before launch, inspect the limits at the mailbox, tenant or subscription, hourly and daily levels. Record each counter's unit, time window and what happens when it is reached.

The external message limit covers the maximum external recipients per hour.⁵ The internal message limit covers the maximum internal recipients per hour.⁶ The daily message limit covers the maximum total recipients per day.⁷ Reaching any configured limit can prevent the user from sending messages.⁸

Provider caps can vary by sending setup. In Azure Communication Services, Custom-domain Send Email is limited to 30 emails per subscription per minute, with higher limits available.⁹ Custom-domain Send Email is also limited to 100 emails per subscription per 60 minutes, with higher limits available.¹⁰ Azure-managed-domain Send Email is limited to 10 emails per subscription per 60 minutes, with no higher limit available.¹¹

Use the provider's own counters for capacity planning.

Launch a new mailbox or domain

Treat a new sending identity as unproven capacity. Start at the launch pace, watch how it handles normal sending, and raise the pace on a schedule after it has built sending history.

New domains have no reputation history, and mailbox providers monitor them closely.¹² Certain mailbox providers hold senders to daily delivery thresholds until the senders establish a reputation.¹³ For Microsoft 365 outbound, warm each inbox before sending to cold contacts and verify the email list before sending.¹⁴

Increase the pace after the mailbox has handled its assigned volume cleanly and the domain can carry the next increment. Keep each increase gradual enough that a problem points to a manageable change in volume.

Calculate the mailbox count

Turn the campaign target into infrastructure before contacts enter a sequence. Use the safe mailbox target in the calculation so the result remains usable when the provider's technical maximum is higher.

The formula is: target mailboxes = monthly email target / sending days per month / safe daily limit per mailbox.^{15, 16}

For example, two domains with three mailboxes each, sending 20 messages per mailbox per day, produce 120 sends per day.¹⁷ If the result does not cover the campaign target, add capacity and keep the pace per mailbox within the chosen target.

Scale across infrastructure

Scale by adding capacity with its own sending history. Spread the load across mailboxes and domains so a problem in one sending identity does not carry the whole campaign.

When cold outreach exceeds the caps per inbox and per tenant, distribute it across independently warmed inboxes and separate sending domains with separate reputations.¹⁸ Increase volume through more inboxes across more domains, while holding each inbox at a human-looking volume and ramping it on a schedule.¹⁹

Review the domain count as well as the mailbox count. A mailbox plan can fit its per-inbox target and still place too much total volume on one domain.

Monitor and adjust

Review the counters that can stop delivery and the usage patterns that show load building. Check each mailbox and domain before adding volume.

Monitor overall, per-content and per-recipient email use.²⁰ Keep the quota math honest: a message sent to an address on the account-level suppression list still counts toward the daily sending quota.²¹

The "Email sending limit exceeded" alert fires when someone sends more mail than the outbound spam policy allows.²² The "Tenant restricted from sending unprovisioned email" alert fires when too much email is sent from unregistered domains.²³ Use either alert as a pause trigger. Inspect the sending identity, recent volume and campaign list before adding load again.

What not to do

• Do not treat messages sent to addresses on the account-level suppression list as free capacity; they count toward the daily quota.²¹
• If you need more headroom, request higher sending-volume limits before reaching the current limit.²⁴
• Build reputation slowly to enable later scaling.²⁵ Rushing can require weeks of recovery.²⁶
• Do not start with fewer mailboxes when the campaign already needs more capacity. Doing so limits daily sending capacity and raises the risk of quickly reaching spam thresholds.²⁷