diff --git a/.pylintrc b/.pylintrc
index 041c69d..c5b3cdd 100644
--- a/.pylintrc
+++ b/.pylintrc
@@ -441,7 +441,9 @@ disable=raw-checker-failed,
         singleton-comparison,
         missing-module-docstring,
         missing-class-docstring,
-        missing-function-docstring
+        missing-function-docstring,
+        too-many-arguments,
+        too-many-locals,
 
 # Enable the message, report, category or checker with the given id(s). You can
 # either give multiple identifier separated by comma (,) or put this option
diff --git a/README.md b/README.md
index 73464d3..6e48daa 100644
--- a/README.md
+++ b/README.md
@@ -22,13 +22,13 @@ Internally, a HTTP API is exposed, and a job queue is used to distribute the che
 - [X] donner un aperçu rapide de l’état de la supervision.
 - [ ] Use background tasks for alerting (#23)
 - [ ] Delete outdated tasks from config (#19, !25)
-- [ ] Implement alerting tasks (#15, 16, !13)
-- [ ] Handles multiple alerting backends (email, sms, gotify) (!13)
+- [X] Implement alerting tasks (#15, 16, !13)
+- [X] Handles multiple alerting backends (email, sms, gotify) (!13)
 - [ ] add an "unknown" severity for check errors (!17)
 - [ ] Add a command to generate new authentication token (#22)
 - [ ] Add a way to specify the severity of the alerts in the config
 - [ ] Allow passing a dict to check
-- [ ] Un flag de configuration permet d’ajouter automatiquement un job de vérification de redirection 301 de la version HTTP vers HTTPS
+- [ ] A configuration flag can automatically add a check of 301 redirection from HTTP to HTTPS
 
 ## License
 
diff --git a/argos/schemas/config.py b/argos/schemas/config.py
index e7a7825..160b5ae 100644
--- a/argos/schemas/config.py
+++ b/argos/schemas/config.py
@@ -4,7 +4,15 @@ For database models, see argos.server.models.
 """
 from typing import Dict, List, Literal, Optional, Tuple
 
-from pydantic import BaseModel, ConfigDict, HttpUrl, field_validator
+from pydantic import (
+    BaseModel,
+    ConfigDict,
+    HttpUrl,
+    StrictBool,
+    EmailStr,
+    PositiveInt,
+    field_validator,
+)
 from pydantic.functional_validators import BeforeValidator
 from typing_extensions import Annotated
 
@@ -93,6 +101,23 @@ class Service(BaseModel):
     secrets: List[str]
 
 
+class MailAuth(BaseModel):
+    """Mail authentication configuration"""
+    login: str
+    password: str
+
+
+class Mail(BaseModel):
+    """Mail configuration"""
+    mailfrom: EmailStr
+    host: str = '127.0.0.1'
+    port: PositiveInt = 25
+    ssl: StrictBool = False
+    starttls: StrictBool = False
+    auth: Optional[MailAuth] = None
+    addresses: List[EmailStr]
+
+
 class Alert(BaseModel):
     """List of way to handle alerts, by severity"""
 
@@ -102,11 +127,18 @@ class Alert(BaseModel):
     unknown: List[str]
 
 
+class GotifyUrl(BaseModel):
+    url: HttpUrl
+    tokens: List[str]
+
+
 class General(BaseModel):
     """Frequency for the checks and alerts"""
 
     frequency: int
     alerts: Alert
+    mail: Optional[Mail] = None
+    gotify: Optional[List[GotifyUrl]] = None
 
     @field_validator("frequency", mode="before")
     def parse_frequency(cls, value):
diff --git a/argos/server/alerting.py b/argos/server/alerting.py
index 5f5821a..7c07533 100644
--- a/argos/server/alerting.py
+++ b/argos/server/alerting.py
@@ -1,9 +1,125 @@
+import ssl
+import smtplib
+
+from typing import List
+from urllib.parse import urlparse
+
+import httpx
+
+from argos.checks.base import Severity
 from argos.logging import logger
+from argos.schemas.config import Config, Mail, GotifyUrl
 
 # XXX Implement mail alerts https://framagit.org/framasoft/framaspace/argos/-/issues/15
 # XXX Implement gotify alerts https://framagit.org/framasoft/framaspace/argos/-/issues/16
 
-def handle_alert(config, result, task, severity):
+def handle_alert(config: Config, result, task, severity, old_severity, request):
     """Dispatch alert through configured alert channels"""
-    msg = f"task={task.id}, status={result.status}, {severity=}"
-    logger.error("Alerting stub: %s", msg)
+
+    if 'local' in getattr(config.general.alerts, severity):
+        logger.error("Alerting stub: task=%i, status=%s, severity=%s",
+                     task.id,
+                     result.status,
+                     severity)
+
+    if config.general.mail is not None and \
+        'mail' in getattr(config.general.alerts, severity):
+        notify_by_mail(result, task, severity, old_severity, config.general.mail, request)
+
+    if config.general.gotify is not None and \
+        'gotify' in getattr(config.general.alerts, severity):
+        notify_with_gotify(result, task, severity, old_severity, config.general.gotify, request)
+
+
+def notify_by_mail(result, task, severity: str, old_severity: str, config: Mail, request) -> None:
+    logger.debug('Will send mail notification')
+
+    msg = f"""\
+URL:    {task.url}
+Check:  {task.check}
+Status: {severity}
+Time:   {result.submitted_at}
+Previous status: {old_severity}
+
+See results of task on {request.url_for('get_task_results', task_id=task.id)}
+"""
+
+    mail = f"""\
+Subject: [Argos] {urlparse(task.url).netloc}: status {severity}
+
+{msg}"""
+
+    if config.ssl:
+        logger.debug('Mail notification: SSL')
+        context = ssl.create_default_context()
+        smtp = smtplib.SMTP_SSL(host=config.host,
+                                port=config.port,
+                                context=context)
+    else:
+        smtp = smtplib.SMTP(host=config.host,  # type: ignore
+                            port=config.port)
+        if config.starttls:
+            logger.debug('Mail notification: STARTTLS')
+            context = ssl.create_default_context()
+            smtp.starttls(context=context)
+
+    if config.auth is not None:
+        logger.debug('Mail notification: authentification')
+        smtp.login(config.auth.login,
+                   config.auth.password)
+
+    for address in config.addresses:
+        logger.debug('Sending mail to %s', address)
+        logger.debug(msg)
+        smtp.sendmail(config.mailfrom, address, mail)
+
+
+def notify_with_gotify(
+    result,
+    task,
+    severity: str,
+    old_severity: str,
+    config: List[GotifyUrl],
+    request
+) -> None:
+    logger.debug('Will send gotify notification')
+    headers = {'accept': 'application/json',
+               'content-type': 'application/json'}
+
+    priority = 9
+    icon = '❌'
+    if severity == Severity.OK:
+        priority = 1
+        icon = '✅'
+    elif severity == Severity.WARNING:
+        priority = 5
+        icon = '⚠️'
+
+    subject = f"{icon} {urlparse(task.url).netloc}: status {severity}"
+    msg = f"""\
+URL:    {task.url}
+Check:  {task.check}
+Status: {severity}
+Time:   {result.submitted_at}
+Previous status: {old_severity}
+
+See results of task on {request.url_for('get_task_results', task_id=task.id)}
+"""
+
+    payload = {'title': subject,
+               'message': msg,
+               'priority': priority}
+
+    for url in config:
+        logger.debug('Sending gotify message(s) to %s', url)
+        for token in url.tokens:
+            try:
+                res = httpx.post(f"{url.url}message",
+                                 params={'token': token},
+                                 headers=headers,
+                                 json=payload)
+                res.raise_for_status()
+            except httpx.RequestError as err:
+                logger.error('An error occurred while sending a message to %s with token %s',
+                             err.request.url,
+                             token)
diff --git a/argos/server/routes/api.py b/argos/server/routes/api.py
index 57001c6..244f499 100644
--- a/argos/server/routes/api.py
+++ b/argos/server/routes/api.py
@@ -44,19 +44,23 @@ async def create_results(
     agent_id = agent_id or request.client.host
     db_results = []
     for agent_result in results:
-        result = await queries.create_result(db, agent_result, agent_id)
         # XXX Maybe offload this to a queue.
         # XXX Get all the tasks at once, to limit the queries on the db
         task = await queries.get_task(db, agent_result.task_id)
         if not task:
             logger.error("Unable to find task %i", agent_result.task_id)
         else:
+            last_severity = task.severity
+            result = await queries.create_result(db, agent_result, agent_id)
             check = task.get_check()
             status, severity = await check.finalize(config, result, **result.context)
             result.set_status(status, severity)
             task.set_times_and_deselect()
 
-            handle_alert(config, result, task, severity)
+            # Don’t create an alert if the severity has not changed
+            if last_severity != severity:
+                # XXX Use a job queue or make it async
+                handle_alert(config, result, task, severity, last_severity, request)
 
         db_results.append(result)
     db.commit()
diff --git a/argos/server/templates/index.html b/argos/server/templates/index.html
index 9d347ef..e1b41bf 100644
--- a/argos/server/templates/index.html
+++ b/argos/server/templates/index.html
@@ -12,7 +12,7 @@
                     {{ counts_dict['ok'] }}
                 </article>
                 <article>
-                    <header> ⚠️ Warning</header>
+                    <header>⚠️ Warning</header>
                     {{ counts_dict['warning'] }}
                 </article>
                 <article>
diff --git a/config-example.yaml b/config-example.yaml
index d2afa2a..8bbf908 100644
--- a/config-example.yaml
+++ b/config-example.yaml
@@ -10,6 +10,24 @@ general:
       - local
     unknown:
       - local
+#  mail:
+#    mailfrom: no-reply@example.org
+#    host: 127.0.0.1
+#    port: 25
+#    ssl: False
+#    starttls: False
+#    auth:
+#      login: foo
+#      password: bar
+#    addresses:
+#      - foo@admin.example.org
+#      - bar@admin.example.org
+#  gotify:
+#    - url: https://example.org
+#      tokens:
+#        - foo
+#        - bar
+
 service:
   secrets:
     # Secrets can be generated using `openssl rand -base64 32`.
diff --git a/pyproject.toml b/pyproject.toml
index f4a47ff..67da05e 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -20,7 +20,7 @@ dependencies = [
   "click>=8.1,<9",
   "fastapi>=0.103,<0.104",
   "httpx>=0.25,<1",
-  "pydantic>=2.4,<3",
+  "pydantic[email]>=2.4,<3",
   "pyyaml>=6.0,<7",
   "pyyaml-include>=1.3,<2",
   "sqlalchemy[asyncio]>=2.0,<3",