Deliverability: throttling and warmup

Applies automatically to every mailing — no setup required, but tunable

Two layered safeguards that protect your sender reputation when you ship at volume:

1. Per-recipient-domain throttling — pace inbound traffic to each receiver (Gmail, Outlook, Yahoo, …) at the rate that provider tolerates from an unauthenticated sender.
2. Sender-domain warmup — new verified sender domains start at a low daily send cap and ramp linearly to full speed over the first 30 days.

Both run inside MailingSenderService. Both are advisory bounds — recipients blocked by either gate are deferred to the next sweep / next UTC day, not lost.

Per-recipient-domain throttling

Token-bucket per destination domain. Each bucket starts full at the configured capacity and refills continuously at capacity / 60 tokens per second. When the bucket is empty, that recipient defers and the mailing flips back to Status = queued so the next 10s sweep picks it up.

Defaults

Override

In appsettings.json:

{
  "Mailing": {
    "Throttle": {
      "DefaultPerMinute": 60,
      "PerDomain": {
        "gmail.com": 150,
        "yourcompany.com": 0
      }
    }
  }
}

• DefaultPerMinute = 0 — disable throttling globally
• PerDomain.<host> = 0 — skip throttling for that one domain (useful for self-hosted internal MTAs where you control the inbound side)

Sender-domain warmup

When you verify a new sender domain, receivers have no history of you. A 5,000-recipient blast on day 1 looks identical to a spam-bot spinning up a fresh domain. Warmup ramps you up gradually so your reputation builds.

How the ramp works

Day-N daily cap = DailyCapDay1 * N, where N is the number of full days since your domain's DkimVerifiedAt timestamp.

With the defaults (DailyCapDay1 = 50, RampDays = 30):

Counter resets at UTC midnight per sender domain. Recipients beyond today's cap defer to tomorrow — the mailing's Status flips back to queued so the regular sender loop ships them then.

Override

{
  "Mailing": {
    "Warmup": {
      "Enabled": true,
      "DailyCapDay1": 50,
      "RampDays": 30
    }
  }
}

Set Enabled = false to disable warmup entirely (sends rely on throttling alone). Smaller DailyCapDay1 for more conservative ramps; larger if you have evidence your sender reputation is already healthy.

When warmup applies

Skipped when:

• The mailing has no verified sender domain (platform default sender already has reputation — warmup would punish legitimate sends).
• The sender domain's DkimVerifiedAt is null (DKIM not actually signing yet).
• More than RampDays days have passed since DkimVerifiedAt.

Applies when sending from a verified SenderDomain row whose DKIM was verified within the last RampDays days.

How they layer

Both gates check on every recipient. The lower of the two wins:

• New sender on day 1 — warmup caps you at 50/day total. Even Gmail's 100/min throttle won't let you ship more, because warmup runs first.
• Established sender shipping to Gmail in a 5,000-recipient blast — warmup doesn't apply (day 30+), so Gmail's 100/min throttle paces you to roughly 6,000 sends/hour to gmail.com specifically.
• Mixed receiver mailing in the middle of a warmup — both apply, you ship the warmup-bound count split across receiver buckets each minute.

Worked example — new newsletter sender

Mira just verified news.miracultureletter.com with DKIM 5 minutes ago. Her list is 8,400 contacts.

1. Day 1: she queues the first issue. Warmup cap = 50. The first 50 recipients ship across the next ~1 minute (per-receiver throttle paces inside that minute). Remaining 8,350 defer to day 2.
2. Day 2: cap = 100. 100 ship today. 8,250 defer.
3. …
4. Day 30: cap = 1,500. By now Gmail and Outlook have seen Mira's signature ~30 days in a row at modest volumes — sender reputation is healthy.
5. Day 31+: warmup off, only the per-receiver throttle paces sends.

Mira didn't have to think about any of this — both gates run automatically the moment a verified sender domain is in scope.

Related

• Sender domain — verify SPF + DKIM + DMARC, enable signing
• Spam-score check — pre-send content review
• Email Mailing — compose basics