diff --git a/app/custom_domain_utils.py b/app/custom_domain_utils.py
index 66a7ded8..d54760b5 100644
--- a/app/custom_domain_utils.py
+++ b/app/custom_domain_utils.py
@@ -3,15 +3,16 @@ import re
 
 from dataclasses import dataclass
 from enum import Enum
-from typing import Optional
+from typing import List, Optional
 
 from app.config import JOB_DELETE_DOMAIN
 from app.db import Session
 from app.email_utils import get_email_domain_part
 from app.log import LOG
-from app.models import User, CustomDomain, SLDomain, Mailbox, Job
+from app.models import User, CustomDomain, SLDomain, Mailbox, Job, DomainMailbox
 
 _ALLOWED_DOMAIN_REGEX = re.compile(r"^(?!-)[A-Za-z0-9-]{1,63}(?<!-)$")
+_MAX_MAILBOXES_PER_DOMAIN = 20
 
 
 @dataclass
@@ -45,6 +46,20 @@ class CannotUseDomainReason(Enum):
             raise Exception("Invalid CannotUseDomainReason")
 
 
+class CannotSetCustomDomainMailboxesCause(Enum):
+    InvalidMailbox = "Something went wrong, please retry"
+    NoMailboxes = "You must select at least 1 mailbox"
+    TooManyMailboxes = (
+        f"You can only set up to {_MAX_MAILBOXES_PER_DOMAIN} mailboxes per domain"
+    )
+
+
+@dataclass
+class SetCustomDomainMailboxesResult:
+    success: bool
+    reason: Optional[CannotSetCustomDomainMailboxesCause] = None
+
+
 def is_valid_domain(domain: str) -> bool:
     """
     Checks that a domain is valid according to RFC 1035
@@ -140,3 +155,40 @@ def delete_custom_domain(domain: CustomDomain):
         run_at=arrow.now(),
         commit=True,
     )
+
+
+def set_custom_domain_mailboxes(
+    user_id: int, custom_domain: CustomDomain, mailbox_ids: List[int]
+) -> SetCustomDomainMailboxesResult:
+    if len(mailbox_ids) == 0:
+        return SetCustomDomainMailboxesResult(
+            success=False, reason=CannotSetCustomDomainMailboxesCause.NoMailboxes
+        )
+    elif len(mailbox_ids) > _MAX_MAILBOXES_PER_DOMAIN:
+        return SetCustomDomainMailboxesResult(
+            success=False, reason=CannotSetCustomDomainMailboxesCause.TooManyMailboxes
+        )
+
+    mailboxes = (
+        Session.query(Mailbox)
+        .filter(
+            Mailbox.id.in_(mailbox_ids),
+            Mailbox.user_id == user_id,
+            Mailbox.verified == True,  # noqa: E712
+        )
+        .all()
+    )
+    if len(mailboxes) != len(mailbox_ids):
+        return SetCustomDomainMailboxesResult(
+            success=False, reason=CannotSetCustomDomainMailboxesCause.InvalidMailbox
+        )
+
+    # first remove all existing domain-mailboxes links
+    DomainMailbox.filter_by(domain_id=custom_domain.id).delete()
+    Session.flush()
+
+    for mailbox in mailboxes:
+        DomainMailbox.create(domain_id=custom_domain.id, mailbox_id=mailbox.id)
+
+    Session.commit()
+    return SetCustomDomainMailboxesResult(success=True)
diff --git a/app/dashboard/views/domain_detail.py b/app/dashboard/views/domain_detail.py
index 73909812..0911a748 100644
--- a/app/dashboard/views/domain_detail.py
+++ b/app/dashboard/views/domain_detail.py
@@ -7,7 +7,7 @@ from wtforms import StringField, validators, IntegerField
 
 from app.constants import DMARC_RECORD
 from app.config import EMAIL_SERVERS_WITH_PRIORITY, EMAIL_DOMAIN
-from app.custom_domain_utils import delete_custom_domain
+from app.custom_domain_utils import delete_custom_domain, set_custom_domain_mailboxes
 from app.custom_domain_validation import CustomDomainValidation
 from app.dashboard.base import dashboard_bp
 from app.db import Session
@@ -16,7 +16,6 @@ from app.models import (
     Alias,
     DomainDeletedAlias,
     Mailbox,
-    DomainMailbox,
     AutoCreateRule,
     AutoCreateRuleMailbox,
 )
@@ -220,40 +219,16 @@ def domain_detail(custom_domain_id):
             )
         elif request.form.get("form-name") == "update":
             mailbox_ids = request.form.getlist("mailbox_ids")
-            # check if mailbox is not tempered with
-            mailboxes = []
-            for mailbox_id in mailbox_ids:
-                mailbox = Mailbox.get(mailbox_id)
-                if (
-                    not mailbox
-                    or mailbox.user_id != current_user.id
-                    or not mailbox.verified
-                ):
-                    flash("Something went wrong, please retry", "warning")
-                    return redirect(
-                        url_for(
-                            "dashboard.domain_detail", custom_domain_id=custom_domain.id
-                        )
-                    )
-                mailboxes.append(mailbox)
+            result = set_custom_domain_mailboxes(
+                user_id=current_user.id,
+                custom_domain=custom_domain,
+                mailbox_ids=mailbox_ids,
+            )
 
-            if not mailboxes:
-                flash("You must select at least 1 mailbox", "warning")
-                return redirect(
-                    url_for(
-                        "dashboard.domain_detail", custom_domain_id=custom_domain.id
-                    )
-                )
-
-            # first remove all existing domain-mailboxes links
-            DomainMailbox.filter_by(domain_id=custom_domain.id).delete()
-            Session.flush()
-
-            for mailbox in mailboxes:
-                DomainMailbox.create(domain_id=custom_domain.id, mailbox_id=mailbox.id)
-
-            Session.commit()
-            flash(f"{custom_domain.domain} mailboxes has been updated", "success")
+            if result.success:
+                flash(f"{custom_domain.domain} mailboxes has been updated", "success")
+            else:
+                flash(result.reason.value, "warning")
 
             return redirect(
                 url_for("dashboard.domain_detail", custom_domain_id=custom_domain.id)
diff --git a/tests/test_custom_domain_utils.py b/tests/test_custom_domain_utils.py
index af6ab70e..ec60f896 100644
--- a/tests/test_custom_domain_utils.py
+++ b/tests/test_custom_domain_utils.py
@@ -7,11 +7,13 @@ from app.custom_domain_utils import (
     create_custom_domain,
     is_valid_domain,
     sanitize_domain,
+    set_custom_domain_mailboxes,
     CannotUseDomainReason,
+    CannotSetCustomDomainMailboxesCause,
 )
 from app.db import Session
-from app.models import User, CustomDomain, Mailbox
-from tests.utils import get_proton_partner
+from app.models import User, CustomDomain, Mailbox, DomainMailbox
+from tests.utils import get_proton_partner, random_email
 from tests.utils import create_new_user, random_string, random_domain
 
 user: Optional[User] = None
@@ -147,3 +149,119 @@ def test_creates_custom_domain_with_partner_id():
     assert res.instance.domain == domain
     assert res.instance.user_id == user.id
     assert res.instance.partner_id == proton_partner.id
+
+
+# set_custom_domain_mailboxes
+def test_set_custom_domain_mailboxes_empty_list():
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+    res = set_custom_domain_mailboxes(user.id, domain, [])
+    assert res.success is False
+    assert res.reason == CannotSetCustomDomainMailboxesCause.NoMailboxes
+
+
+def test_set_custom_domain_mailboxes_mailbox_from_another_user():
+    other_user = create_new_user()
+    other_mailbox = Mailbox.create(
+        user_id=other_user.id, email=random_email(), verified=True
+    )
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+
+    res = set_custom_domain_mailboxes(user.id, domain, [other_mailbox.id])
+    assert res.success is False
+    assert res.reason == CannotSetCustomDomainMailboxesCause.InvalidMailbox
+
+
+def test_set_custom_domain_mailboxes_mailbox_from_current_user_and_another_user():
+    other_user = create_new_user()
+    other_mailbox = Mailbox.create(
+        user_id=other_user.id, email=random_email(), verified=True
+    )
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+
+    res = set_custom_domain_mailboxes(
+        user.id, domain, [user.default_mailbox_id, other_mailbox.id]
+    )
+    assert res.success is False
+    assert res.reason == CannotSetCustomDomainMailboxesCause.InvalidMailbox
+
+
+def test_set_custom_domain_mailboxes_success():
+    other_mailbox = Mailbox.create(user_id=user.id, email=random_email(), verified=True)
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+
+    res = set_custom_domain_mailboxes(
+        user.id, domain, [user.default_mailbox_id, other_mailbox.id]
+    )
+    assert res.success is True
+    assert res.reason is None
+
+    domain_mailboxes = DomainMailbox.filter_by(domain_id=domain.id).all()
+    assert len(domain_mailboxes) == 2
+    assert domain_mailboxes[0].domain_id == domain.id
+    assert domain_mailboxes[0].mailbox_id == user.default_mailbox_id
+    assert domain_mailboxes[1].domain_id == domain.id
+    assert domain_mailboxes[1].mailbox_id == other_mailbox.id
+
+
+def test_set_custom_domain_mailboxes_set_twice():
+    other_mailbox = Mailbox.create(user_id=user.id, email=random_email(), verified=True)
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+
+    res = set_custom_domain_mailboxes(
+        user.id, domain, [user.default_mailbox_id, other_mailbox.id]
+    )
+    assert res.success is True
+    assert res.reason is None
+
+    res = set_custom_domain_mailboxes(
+        user.id, domain, [user.default_mailbox_id, other_mailbox.id]
+    )
+    assert res.success is True
+    assert res.reason is None
+
+    domain_mailboxes = DomainMailbox.filter_by(domain_id=domain.id).all()
+    assert len(domain_mailboxes) == 2
+    assert domain_mailboxes[0].domain_id == domain.id
+    assert domain_mailboxes[0].mailbox_id == user.default_mailbox_id
+    assert domain_mailboxes[1].domain_id == domain.id
+    assert domain_mailboxes[1].mailbox_id == other_mailbox.id
+
+
+def test_set_custom_domain_mailboxes_removes_old_association():
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain(), commit=True)
+
+    res = set_custom_domain_mailboxes(user.id, domain, [user.default_mailbox_id])
+    assert res.success is True
+    assert res.reason is None
+
+    other_mailbox = Mailbox.create(
+        user_id=user.id, email=random_email(), verified=True, commit=True
+    )
+    res = set_custom_domain_mailboxes(user.id, domain, [other_mailbox.id])
+    assert res.success is True
+    assert res.reason is None
+
+    domain_mailboxes = DomainMailbox.filter_by(domain_id=domain.id).all()
+    assert len(domain_mailboxes) == 1
+    assert domain_mailboxes[0].domain_id == domain.id
+    assert domain_mailboxes[0].mailbox_id == other_mailbox.id
+
+
+def test_set_custom_domain_mailboxes_with_unverified_mailbox():
+    domain = CustomDomain.create(user_id=user.id, domain=random_domain())
+    verified_mailbox = Mailbox.create(
+        user_id=user.id,
+        email=random_email(),
+        verified=True,
+    )
+    unverified_mailbox = Mailbox.create(
+        user_id=user.id,
+        email=random_email(),
+        verified=False,
+    )
+
+    res = set_custom_domain_mailboxes(
+        user.id, domain, [verified_mailbox.id, unverified_mailbox.id]
+    )
+    assert res.success is False
+    assert res.reason is CannotSetCustomDomainMailboxesCause.InvalidMailbox