How SMTP Relay sending limits work

An SMTP Relay service has two sending limits. The monthly allowance is how many emails the relay may send in a calendar month, and the plan sets it. The hourly limit is how many emails the relay sends in an hour. It starts small on every new relay and rises as the relay builds a clean sending history. Both are on the service's Overview tab in the dashboard.

The free plan includes 3,000 emails a month and stops there. Each paid plan includes more and adds a 12% buffer: the relay can send 12% more than the plan's number before sending stops. On a plan of 25,000 emails a month, the allowance is 28,000. The Mail page lists the plans and their prices.

We count like this:

• Each message we accept counts once, however many recipients it has. The send API sends a marketing message as one copy for each recipient, so each copy counts.
• A message counts when we accept it, whether it is later delivered, deferred or bounced. A message we refuse does not count.
• The count starts again at 00:00 UTC on the 1st of each month.
• We add up accepted messages once an hour, so the count trails your sending by up to about an hour.

The first figure on the Overview tab is Emails this month, or Emails this month (with buffer) on a paid plan. It shows the count against the allowance and when it resets. A relay that our staff set up with no monthly cap shows No monthly cap.

As the count rises, we email you at your account's address, and the Overview tab shows an alert:

The subject starts with the relay's service ID. We send each email once a month. When the count passes several points between two counts, we send only the email for the highest. While the relay sends from its buffer, the alert says how many emails the buffer still covers.

Once the count reaches the allowance, we refuse new mail:

• Over SMTP, the relay answers 550 5.7.1 Monthly allowance and its 12% buffer are used up; upgrade the plan to continue. On the free plan it answers 550 5.7.1 Monthly sending allowance reached; upgrade the plan to continue.
• The send API answers 429 with the same text in detail.

Both check the count plus the recipients of the message in hand. Near the end of the allowance, a message to many recipients can be refused while a message to one recipient still goes out. Because the count trails by up to about an hour, a relay can go past its allowance by up to about an hour of sending before we refuse mail.

Sending starts again at 00:00 UTC on the 1st. Each alert has Change plan, which opens the service's Billing tab. The plan-change dialog there only moves a service between billing cycles of the same plan, and each relay plan is a plan of its own, so it cannot move the relay to a larger plan. To raise the allowance before the month ends, contact support.

The hourly limit caps how many emails the relay sends in an hour, which protects the reputation of the addresses it shares with other senders. A new relay starts with a small limit, and a daily review raises it one step at a time. The steps are 100, 250, 500, 1,000, 2,500, 5,000, 10,000, 25,000 and 50,000 emails an hour.

Mail over the limit waits in the queue and goes out in a later hour. While it waits, the Events tab can show deferred events for it. If mail keeps arriving faster than the limit, the queue grows, and once six hours of mail at the hourly limit is waiting, we refuse new messages until the queue shrinks.

The review runs once a day and raises the limit one step when all of these are true:

• The relay has sent clean mail for its first 14 days, or has sent 5,000 clean messages sooner. Clean means the next rule holds.
• The relay is not under review, and its sending is not paused.
• Over the last 14 days, fewer than 2% of the messages it sent bounced and fewer than 0.1% drew a complaint.
• The relay has had its current limit for at least 2 days.
• In the last 14 days it sent at least 12 times its hourly limit, such as 1,200 messages at 100 an hour.
• The next step is not above the plan's automatic ceiling.

A raised limit takes effect within about an hour.

The automatic ceiling is the highest step at or below the plan's monthly emails divided by 25: a rate at which the relay could send the plan's monthly emails in about a day. A relay with no monthly cap can climb every step.

On the free plan, the limit does not rise above 100 on its own. Once the limit reaches the ceiling, the Hourly limit card says This plan's automatic ceiling is reached; ask for more below. To go higher, request a higher limit: our staff can set any limit, above the top step too. Changing the plan does not change the hourly limit, only the ceiling the review can reach.

The limit can also go down. When a relay goes under review or has its sending paused, or when 2% of its messages in the last 14 days bounce or 0.1% draw a complaint, the next daily review can set its limit back to the starting limit.

On the Overview tab, the Hourly limit card shows:

Current limit

The limit now, such as 250 emails an hour.

Next step

The step the review raises the limit to next. The card leaves it out once the next step is above the plan's ceiling.

Set

How and when the current limit was set: Starting limit, Raised automatically, Set by support or Reset after a reputation review, then how long ago.

Request limit increase… asks our staff for more. See Request a higher hourly limit.

A relay sends from addresses it shares with other relays unless it has a dedicated address: an IPv4 address that only your relay sends from, with a reverse DNS name of its own. Its reputation depends on your mail alone.

• Plans of 250,000 emails a month and more include one dedicated address.
• On plans of 100,000 emails a month and more, you can add dedicated addresses for $50 a month each. Each one you add is a service of its own, with its own invoice (Dedicated IP add-ons).

You add one with Order dedicated IP on the Sending IPs card of the Overview tab, which opens the address's order page (Order a dedicated sending IP). On a smaller plan the button is greyed out, and the card says which plan you need.

Until we have assigned an address, the card says We are assigning your dedicated addresses. Then it lists each address with its reverse DNS name and its current hourly cap, such as 500/h. A Warming up badge shows until the address reaches its last stage (Warm-up). An address you bought on its own has an Add-on badge, and one that is turned off has a Disabled badge. From then on, the relay's mail leaves from its dedicated addresses.

When you cancel a dedicated address you bought on its own and it ends, we release the address. When a relay's last dedicated address goes, its mail goes back to the shared addresses. Cancelling the relay does not cancel the addresses you bought on their own, so cancel those too.

Mail providers distrust a new address that suddenly sends a lot. So each dedicated address starts with a small hourly cap, which rises in stages:

We check once a day. An address moves up one stage when it has spent at least 2 days in its stage, sent the emails the table gives, and kept bounces under 2% and complaints under 0.1% in that stage. We can also hold every address in a region at its stage for a while, when large providers are filtering mail from that region. From stage 8, only the relay's hourly limit applies.

During warm-up, mail to Gmail, Microsoft (Outlook.com, Hotmail, Live and MSN), Yahoo (with AOL) and Apple (iCloud) has a cap of its own at each address: a quarter of the address's hourly cap, and at least 10.

Mail over a cap waits in the queue, as mail over the hourly limit does. A warming address can have a lower cap than the relay's hourly limit, so plan large sends for after the warm-up.

GET /client/smtp-relay/{service_id}/usage returns the month's count, a daily series and the relay's limits:

curl "https://api.coritan.com/api/v1/client/smtp-relay/4812/usage?days=3" \
  -H "Authorization: Bearer $CORITAN_TOKEN"

days is how far back the series goes, 1–365 days, and 30 when you leave it out. A value outside that range answers 422.

{
  "series": [
    {"day": "2026-09-14", "sent": 1204, "delivered": 1187, "bounced": 6, "deferred": 23, "complaints": 0},
    {"day": "2026-09-15", "sent": 1318, "delivered": 1296, "bounced": 9, "deferred": 31, "complaints": 1},
    {"day": "2026-09-16", "sent": 612, "delivered": 598, "bounced": 3, "deferred": 12, "complaints": 0}
  ],
  "month_to_date_sent": 21460,
  "quota_messages_month": 25000,
  "quota_buffer_pct": 12,
  "monthly_allowance": 28000,
  "limits": {
    "monthly_messages": 25000,
    "quota_buffer_pct": 12,
    "monthly_allowance": 28000,
    "month_to_date_sent": 21460,
    "remaining": 6540,
    "over_quota": false,
    "sending_stopped": false,
    "free_tier": false,
    "domains": null,
    "credentials": null,
    "rate_per_hour": 250,
    "hourly_limit_since": "2026-09-11T04:15:09.482113+00:00",
    "hourly_limit_source": "auto",
    "hourly_limit_next": 500,
    "hourly_limit_auto_ceiling": 1000,
    "dedicated_ip_count": 0,
    "dedicated_ip_addon_eligible": false,
    "dedicated_ip_min_monthly": 100000
  }
}

series

One entry for each day with mail, oldest first, from days days ago up to today (UTC). Each has the day and its counts: sent (messages accepted), delivered, bounced, deferred and complaints. Days with no mail are left out.

month_to_date_sent

Messages accepted since the 1st, as of the last hourly count.

quota_messages_month, quota_buffer_pct, monthly_allowance

The plan's emails a month, the buffer as a percentage, and the allowance they make together. A quota_messages_month of 0 means no monthly cap.

limits

The relay's limits. The service summary, GET /client/smtp-relay/{service_id}, returns the same object.

The fields in limits:

monthly_messages, quota_buffer_pct, monthly_allowance, month_to_date_sent

The same figures as at the top level.

remaining

What is left of the allowance, never below 0. null with no monthly cap.

over_quota

true once the count reaches the plan's number, so the relay is sending from its buffer or has stopped.

sending_stopped

true once the count reaches the allowance.

free_tier

true on the free plan.

domains, credentials

How many sending domains and SMTP credentials the plan allows. null means no cap.

rate_per_hour

The hourly limit.

hourly_limit_since

When the hourly limit was set.

hourly_limit_source

How it was set: start, auto (the daily review), staff or reset.

hourly_limit_next

The next step, or null above the top step. The review raises the limit to it only when it is not above hourly_limit_auto_ceiling.

hourly_limit_auto_ceiling

The highest limit the review reaches on this plan.

dedicated_ip_count

How many dedicated addresses the relay has, from the plan and bought on their own.

dedicated_ip_addon_eligible

true when the plan allows buying more dedicated addresses.

dedicated_ip_min_monthly

The emails a month a plan needs for that: 100000.

The same operation is under GET /client/mail/{service_id}/usage, which answers for both products. On a Mail Hosting service it returns the series and month_to_date_sent for the mail its mailboxes send. Mail Hosting has no monthly allowance, so quota_messages_month, quota_buffer_pct and monthly_allowance are 0, and the answer has no limits.