diff --git a/app/config.py b/app/config.py
index 9ec2cb65..7d6ff975 100644
--- a/app/config.py
+++ b/app/config.py
@@ -62,6 +62,17 @@ def get_env_dict(env_var: str) -> dict[str, str]:
     return result
 
 
+def get_env_csv(env_var: str, default: Optional[str]) -> list[str]:
+    """
+    Get an env variable and convert it into a list of strings separated by,
+    Syntax is: val1,val2
+    """
+    value = os.getenv(env_var, default)
+    if not value:
+        return []
+    return [field.strip() for field in value.split(",") if field.strip()]
+
+
 config_file = os.environ.get("CONFIG")
 if config_file:
     config_file = get_abs_path(config_file)
@@ -171,6 +182,14 @@ FIRST_ALIAS_DOMAIN = os.environ.get("FIRST_ALIAS_DOMAIN") or EMAIL_DOMAIN
 # e.g. [(10, "mx1.hostname."), (10, "mx2.hostname.")]
 EMAIL_SERVERS_WITH_PRIORITY = sl_getenv("EMAIL_SERVERS_WITH_PRIORITY")
 
+PROTON_MX_SERVERS = get_env_csv(
+    "PROTON_MX_SERVERS", "mail.protonmail.ch., mailsec.protonmail.ch."
+)
+
+PROTON_EMAIL_DOMAINS = get_env_csv(
+    "PROTON_EMAIL_DOMAINS", "proton.me, protonmail.com, protonmail.ch, proton.ch, pm.me"
+)
+
 # disable the alias suffix, i.e. the ".random_word" part
 DISABLE_ALIAS_SUFFIX = "DISABLE_ALIAS_SUFFIX" in os.environ
 
diff --git a/app/dns_utils.py b/app/dns_utils.py
index 02ca7841..d2fbd853 100644
--- a/app/dns_utils.py
+++ b/app/dns_utils.py
@@ -115,9 +115,20 @@ class InMemoryDNSClient(DNSClient):
         return self.txt_records.get(hostname, [])
 
 
-def get_network_dns_client() -> NetworkDNSClient:
+global_dns_client: Optional[DNSClient] = None
+
+
+def get_network_dns_client() -> DNSClient:
+    global global_dns_client
+    if global_dns_client is not None:
+        return global_dns_client
     return NetworkDNSClient(NAMESERVERS)
 
 
+def set_global_dns_client(dns_client: Optional[DNSClient]):
+    global global_dns_client
+    global_dns_client = dns_client
+
+
 def get_mx_domains(hostname: str) -> dict[int, list[str]]:
     return get_network_dns_client().get_mx_domains(hostname)
diff --git a/app/models.py b/app/models.py
index 2f572936..3032ba55 100644
--- a/app/models.py
+++ b/app/models.py
@@ -2838,24 +2838,20 @@ class Mailbox(Base, ModelMixin):
         return len(alias_ids)
 
     def is_proton(self) -> bool:
-        if (
-            self.email.endswith("@proton.me")
-            or self.email.endswith("@protonmail.com")
-            or self.email.endswith("@protonmail.ch")
-            or self.email.endswith("@proton.ch")
-            or self.email.endswith("@pm.me")
-        ):
-            return True
+        for proton_email_domain in config.PROTON_EMAIL_DOMAINS:
+            if self.email.endswith(f"@{proton_email_domain}"):
+                return True
 
         from app.email_utils import get_email_local_part
 
         mx_domains = get_mx_domains(get_email_local_part(self.email))
+
+        proton_mx_domains = config.PROTON_MX_SERVERS
         # Proton is the first domain
-        if mx_domains and mx_domains[0].domain in (
-            "mail.protonmail.ch.",
-            "mailsec.protonmail.ch.",
-        ):
-            return True
+        for prio in mx_domains:
+            for mx_domain in mx_domains[prio]:
+                if mx_domain in proton_mx_domains:
+                    return True
 
         return False
 
diff --git a/tests/models/test_mailbox.py b/tests/models/test_mailbox.py
new file mode 100644
index 00000000..68f5d1fe
--- /dev/null
+++ b/tests/models/test_mailbox.py
@@ -0,0 +1,37 @@
+from app import config
+from app.dns_utils import set_global_dns_client, InMemoryDNSClient
+from app.email_utils import get_email_local_part
+from app.models import Mailbox
+from tests.utils import create_new_user, random_email
+
+dns_client = InMemoryDNSClient()
+
+
+def setup_module():
+    set_global_dns_client(dns_client)
+
+
+def teardown_module():
+    set_global_dns_client(None)
+
+
+def test_is_proton_with_email_domain():
+    user = create_new_user()
+    mailbox = Mailbox.create(
+        user_id=user.id, email=f"test@{config.PROTON_EMAIL_DOMAINS[0]}"
+    )
+    assert mailbox.is_proton()
+    mailbox = Mailbox.create(user_id=user.id, email="a@b.c")
+    assert not mailbox.is_proton()
+
+
+def test_is_proton_with_mx_domain():
+    email = random_email()
+    dns_client.set_mx_records(
+        get_email_local_part(email), {10: config.PROTON_MX_SERVERS}
+    )
+    user = create_new_user()
+    mailbox = Mailbox.create(user_id=user.id, email=email)
+    assert mailbox.is_proton()
+    dns_client.set_mx_records(get_email_local_part(email), {10: ["nowhere.net"]})
+    assert not mailbox.is_proton()