diff --git a/api/config/custom-environment-variables.cjs b/api/config/custom-environment-variables.cjs
index 03619174..00592245 100644
--- a/api/config/custom-environment-variables.cjs
+++ b/api/config/custom-environment-variables.cjs
@@ -267,6 +267,10 @@ module.exports = {
     attempts: 'AUTHRATELIMIT_ATTEMPTS',
     duration: 'AUTHRATELIMIT_DURATION'
   },
+  mailsRateLimit: {
+    points: 'MAILSRATELIMIT_POINTS',
+    duration: 'MAILSRATELIMIT_DURATION'
+  },
   oauth: {
     providers: jsonEnv('OAUTH_PROVIDERS'),
     github: {
diff --git a/api/config/default.cjs b/api/config/default.cjs
index 8601d530..3e98ebd6 100644
--- a/api/config/default.cjs
+++ b/api/config/default.cjs
@@ -185,6 +185,12 @@ module.exports = {
     attempts: 5,
     duration: 60
   },
+  // per-recipient mail rate limit, prevents a bug or runaway webhook from spamming a user's mailbox
+  // points = max number of emails per recipient address within the rolling window of `duration` seconds
+  mailsRateLimit: {
+    points: 500,
+    duration: 86400
+  },
   // Example of full oauth provider configuration
   // oauth: {providers: [{
   //   id: 'github',
diff --git a/api/config/test.cjs b/api/config/test.cjs
index 5d75f9c7..4111655e 100644
--- a/api/config/test.cjs
+++ b/api/config/test.cjs
@@ -71,6 +71,11 @@ module.exports = {
     attempts: 100,
     duration: 60
   },
+  mailsRateLimit: {
+    // high enough that no existing test trips it; rate-limit test pre-fills the bucket directly
+    points: 100,
+    duration: 5
+  },
   anonymousAction: {
     // short notBefore keeps tests fast — in prod this is 8s to trip bots
     expiresIn: '1d',
diff --git a/api/config/type/schema.json b/api/config/type/schema.json
index a07d459e..d51fe86b 100644
--- a/api/config/type/schema.json
+++ b/api/config/type/schema.json
@@ -40,6 +40,7 @@
     "saml2",
     "webhooks",
     "authRateLimit",
+    "mailsRateLimit",
     "plannedDeletionDelay",
     "cleanup",
     "avatars",
@@ -482,6 +483,21 @@
         }
       }
     },
+    "mailsRateLimit": {
+      "type": "object",
+      "required": [
+        "points",
+        "duration"
+      ],
+      "properties": {
+        "points": {
+          "type": "integer"
+        },
+        "duration": {
+          "type": "integer"
+        }
+      }
+    },
     "plannedDeletionDelay": {
       "type": "integer"
     },
diff --git a/api/src/mails/service.ts b/api/src/mails/service.ts
index 4d50a041..5481a5f2 100644
--- a/api/src/mails/service.ts
+++ b/api/src/mails/service.ts
@@ -7,6 +7,8 @@ import { flatten } from 'flat'
 import EventEmitter from 'node:events'
 import mailsTransport from './transport.ts'
 import { getSiteByUrl, getSiteByHost } from '#services'
+import { internalError } from '@data-fair/lib-node/observer.js'
+import { mailLimiter } from '../utils/limiter.ts'
 
 export const events = new EventEmitter()
 
@@ -86,6 +88,22 @@ export const sendMailI18n = async (key: string, messages: any, to: string, param
 }
 
 export const sendMail = async (to: string, params: SendMailParams, attachments?: { filename: string, path: string }[]) => {
+  // per-recipient rate limiting — protects users from a bug or runaway webhook spamming their mailbox
+  const recipients = to.split(',').map(r => r.trim().toLowerCase()).filter(r => r.length > 0)
+  const allowed: string[] = []
+  for (const recipient of recipients) {
+    if (await mailLimiter()(recipient)) {
+      allowed.push(recipient)
+    } else {
+      internalError('mail-rate-limited', `recipient ${recipient} exceeded mail rate limit, dropping (subject: ${params.subject})`)
+    }
+  }
+  if (allowed.length === 0) {
+    internalError('mail-rate-limited', `all recipients dropped, mail not sent (subject: ${params.subject}, original to: ${to})`)
+    return
+  }
+  to = allowed.join(', ')
+
   let site = params.link ? (await getSiteByUrl(params.link)) : undefined
   if (!site && params.host) {
     site = await getSiteByHost(params.host, params.path ?? '')
diff --git a/api/src/test-env.ts b/api/src/test-env.ts
index 8bf84255..1180c0bd 100644
--- a/api/src/test-env.ts
+++ b/api/src/test-env.ts
@@ -134,7 +134,9 @@ router.post('/clear-site-cache', async (req, res) => {
 router.get('/config', (req, res) => {
   res.json({
     publicUrl: config.publicUrl,
-    secretKeys: config.secretKeys
+    secretKeys: config.secretKeys,
+    mongo: { url: config.mongo.url },
+    mailsRateLimit: config.mailsRateLimit
   })
 })
 
diff --git a/api/src/utils/limiter.ts b/api/src/utils/limiter.ts
index 5f97f609..c06d58c1 100644
--- a/api/src/utils/limiter.ts
+++ b/api/src/utils/limiter.ts
@@ -4,18 +4,15 @@ import mongo from '#mongo'
 
 // protect authentication routes with rate limiting to prevent brute force attacks
 let _limiter: (key: string) => Promise<boolean> | undefined
-const limiterOptions = {
-  keyPrefix: 'sd-rate-limiter-auth',
-  points: config.authRateLimit.attempts,
-  duration: config.authRateLimit.duration
-}
 
 export default () => {
   if (!_limiter) {
     const rateLimiterMongo = new RateLimiterMongo({
       storeClient: mongo.client,
       dbName: mongo.db.databaseName,
-      ...limiterOptions
+      keyPrefix: 'sd-rate-limiter-auth',
+      points: config.authRateLimit.attempts,
+      duration: config.authRateLimit.duration
     })
     _limiter = async (key: string) => {
       try {
@@ -29,3 +26,28 @@ export default () => {
   }
   return _limiter
 }
+
+// per-recipient mail rate limit, prevents a bug or runaway webhook from spamming a user's mailbox
+let _mailLimiter: (key: string) => Promise<boolean> | undefined
+
+export const mailLimiter = () => {
+  if (!_mailLimiter) {
+    const rateLimiterMongo = new RateLimiterMongo({
+      storeClient: mongo.client,
+      dbName: mongo.db.databaseName,
+      keyPrefix: 'sd-rate-limiter-mail',
+      points: config.mailsRateLimit.points,
+      duration: config.mailsRateLimit.duration
+    })
+    _mailLimiter = async (key: string) => {
+      try {
+        await rateLimiterMongo.consume(key, 1)
+      } catch (err) {
+        if (err instanceof RateLimiterRes) return false
+        throw err
+      }
+      return true
+    }
+  }
+  return _mailLimiter
+}
diff --git a/tests/features/mails-rate-limit.api.spec.ts b/tests/features/mails-rate-limit.api.spec.ts
new file mode 100644
index 00000000..d72cd2b5
--- /dev/null
+++ b/tests/features/mails-rate-limit.api.spec.ts
@@ -0,0 +1,99 @@
+import { strict as assert } from 'node:assert'
+import { test } from '@playwright/test'
+import { axios, testEnvAx, maildevAx, deleteAllEmails, getServerConfig } from '../support/axios.ts'
+import { MongoClient } from 'mongodb'
+import { RateLimiterMongo } from 'rate-limiter-flexible'
+
+// A unique recipient per test process avoids cross-test interference on the same bucket
+const uniqueEmail = (label: string) => `rate-limit-${label}-${Date.now()}-${Math.floor(Math.random() * 1e6)}@test.com`
+
+let mongoClient: MongoClient
+let rateLimiter: RateLimiterMongo
+
+test.describe('mails rate limiting', () => {
+  test.beforeAll(async () => {
+    const cfg = await getServerConfig()
+    mongoClient = await MongoClient.connect(cfg.mongo.url)
+    rateLimiter = new RateLimiterMongo({
+      storeClient: mongoClient,
+      dbName: mongoClient.db().databaseName,
+      keyPrefix: 'sd-rate-limiter-mail',
+      points: cfg.mailsRateLimit.points,
+      duration: cfg.mailsRateLimit.duration
+    })
+    // give the limiter time to ensure its underlying collection exists
+    await rateLimiter.get('warmup')
+  })
+
+  test.afterAll(async () => {
+    if (mongoClient) await mongoClient.close()
+  })
+
+  test.beforeEach(async () => {
+    await deleteAllEmails()
+    await testEnvAx.delete('/')
+    await testEnvAx.post('/seed')
+  })
+
+  test('Delivers email to a fresh recipient (sanity)', async () => {
+    const ax = await axios()
+    const to = uniqueEmail('fresh')
+    const res = await ax.post('/api/mails', {
+      to: [to],
+      subject: 'rate-limit-sanity',
+      text: 'hello'
+    }, { params: { key: 'testkey' } })
+    assert.equal(res.status, 200)
+    await new Promise(resolve => setTimeout(resolve, 100))
+    const emails: any[] = (await maildevAx.get('/email')).data
+    const email = emails.find((m: any) => m.subject === 'rate-limit-sanity')
+    assert.ok(email, 'expected a delivered email for fresh recipient')
+    assert.ok(email.to.some((t: any) => t.address === to), 'expected the fresh recipient to be in the to list')
+  })
+
+  test('Drops email to a recipient whose bucket is already full', async () => {
+    const ax = await axios()
+    const to = uniqueEmail('victim')
+
+    const cfg = await getServerConfig()
+    // pre-fill the rate-limit bucket up to the limit so the next consume() rejects
+    await rateLimiter.set(to.toLowerCase(), cfg.mailsRateLimit.points, cfg.mailsRateLimit.duration * 1000)
+
+    const res = await ax.post('/api/mails', {
+      to: [to],
+      subject: 'rate-limit-victim',
+      text: 'should be dropped'
+    }, { params: { key: 'testkey' } })
+    assert.equal(res.status, 200)
+
+    await new Promise(resolve => setTimeout(resolve, 100))
+    const emails: any[] = (await maildevAx.get('/email')).data
+    const email = emails.find((m: any) => m.subject === 'rate-limit-victim')
+    assert.ok(!email, 'expected no email delivered to a rate-limited recipient')
+  })
+
+  test('Multi-recipient send filters out only the rate-limited address', async () => {
+    const ax = await axios()
+    const victim = uniqueEmail('multi-victim')
+    const fresh = uniqueEmail('multi-fresh')
+
+    const cfg = await getServerConfig()
+    await rateLimiter.set(victim.toLowerCase(), cfg.mailsRateLimit.points, cfg.mailsRateLimit.duration * 1000)
+
+    // a single `to` string with a comma exercises sendMail's multi-recipient split path
+    const res = await ax.post('/api/mails', {
+      to: [`${victim}, ${fresh}`],
+      subject: 'rate-limit-mixed',
+      text: 'partial delivery'
+    }, { params: { key: 'testkey' } })
+    assert.equal(res.status, 200)
+
+    await new Promise(resolve => setTimeout(resolve, 100))
+    const emails: any[] = (await maildevAx.get('/email')).data
+    const email = emails.find((m: any) => m.subject === 'rate-limit-mixed')
+    assert.ok(email, 'expected the email to be delivered with the non-limited recipient')
+    const addresses = email.to.map((t: any) => t.address)
+    assert.ok(addresses.includes(fresh), 'expected the fresh recipient to receive the email')
+    assert.ok(!addresses.includes(victim), 'expected the rate-limited recipient to be filtered out')
+  })
+})