diff --git a/core/admin/mailu/manage.py b/core/admin/mailu/manage.py
index 7f32f6c8..4846c2d6 100644
--- a/core/admin/mailu/manage.py
+++ b/core/admin/mailu/manage.py
@@ -91,7 +91,7 @@ def user(localpart, domain_name, password, hash_scheme=None):
 @click.option('-a', '--max_aliases')
 @click.option('-q', '--max_quota_bytes')
 @flask_cli.with_appcontext
-def domain(domain_name, max_users=0, max_aliases=0, max_quota_bytes=0):
+def domain(domain_name, max_users=-1, max_aliases=-1, max_quota_bytes=0):
     domain = models.Domain.query.get(domain_name)
     if not domain:
         domain = models.Domain(name=domain_name)
@@ -140,8 +140,8 @@ def config_update(verbose=False, delete_objects=False):
         if verbose:
             print(str(domain_config))
         domain_name = domain_config['name']
-        max_users = domain_config.get('max_users', 0)
-        max_aliases = domain_config.get('max_aliases', 0)
+        max_users = domain_config.get('max_users', -1)
+        max_aliases = domain_config.get('max_aliases', -1)
         max_quota_bytes = domain_config.get('max_quota_bytes', 0)
         tracked_domains.add(domain_name)
         domain = models.Domain.query.get(domain_name)
diff --git a/core/admin/mailu/models.py b/core/admin/mailu/models.py
index 30a2ed64..3588b4ae 100644
--- a/core/admin/mailu/models.py
+++ b/core/admin/mailu/models.py
@@ -122,8 +122,8 @@ class Domain(Base):
     name = db.Column(IdnaDomain, primary_key=True, nullable=False)
     managers = db.relationship('User', secondary=managers,
         backref=db.backref('manager_of'), lazy='dynamic')
-    max_users = db.Column(db.Integer, nullable=False, default=0)
-    max_aliases = db.Column(db.Integer, nullable=False, default=0)
+    max_users = db.Column(db.Integer, nullable=False, default=-1)
+    max_aliases = db.Column(db.Integer, nullable=False, default=-1)
     max_quota_bytes = db.Column(db.Integer(), nullable=False, default=0)
     signup_enabled = db.Column(db.Boolean(), nullable=False, default=False)
 
diff --git a/core/admin/mailu/ui/forms.py b/core/admin/mailu/ui/forms.py
index 5ee6da7d..04597872 100644
--- a/core/admin/mailu/ui/forms.py
+++ b/core/admin/mailu/ui/forms.py
@@ -53,8 +53,8 @@ class LoginForm(flask_wtf.FlaskForm):
 
 class DomainForm(flask_wtf.FlaskForm):
     name = fields.StringField(_('Domain name'), [validators.DataRequired()])
-    max_users = fields_.IntegerField(_('Maximum user count'), default=10)
-    max_aliases = fields_.IntegerField(_('Maximum alias count'), default=10)
+    max_users = fields_.IntegerField(_('Maximum user count'), [validators.NumberRange(min=-1)], default=10)
+    max_aliases = fields_.IntegerField(_('Maximum alias count'), [validators.NumberRange(min=-1)], default=10)
     max_quota_bytes = fields_.IntegerSliderField(_('Maximum user quota'), default=0)
     signup_enabled = fields.BooleanField(_('Enable sign-up'), default=False)
     comment = fields.StringField(_('Comment'))
diff --git a/core/admin/mailu/ui/templates/domain/list.html b/core/admin/mailu/ui/templates/domain/list.html
index 0fa54f4f..2431faa5 100644
--- a/core/admin/mailu/ui/templates/domain/list.html
+++ b/core/admin/mailu/ui/templates/domain/list.html
@@ -40,8 +40,8 @@
     {% endif %}
   </td>
   <td>{{ domain.name }}</td>
-  <td>{{ domain.users | count }} / {{ domain.max_users or '∞' }}</td>
-  <td>{{ domain.aliases | count }} / {{ domain.max_aliases or '∞' }}</td>
+  <td>{{ domain.users | count }} / {{ '∞' if domain.max_users == -1 else domain.max_users }}</td>
+  <td>{{ domain.aliases | count }} / {{ '∞' if domain.max_aliases == -1 else domain.max_aliases }}</td>
   <td>{{ domain.comment or '' }}</td>
   <td>{{ domain.created_at }}</td>
   <td>{{ domain.updated_at or '' }}</td>
diff --git a/core/admin/mailu/ui/templates/user/signup_domain.html b/core/admin/mailu/ui/templates/user/signup_domain.html
index 84f33b3d..1cb65ae8 100644
--- a/core/admin/mailu/ui/templates/user/signup_domain.html
+++ b/core/admin/mailu/ui/templates/user/signup_domain.html
@@ -18,7 +18,7 @@
 {% for domain_name, domain in available_domains.items() %}
 <tr>
   <td><a href="{{ url_for('.user_signup', domain_name=domain_name) }}">{{ domain_name }}</a></td>
-  <td>{{ domain.max_users - (domain.users | count) if domain.max_users else '∞' }}</td>
+  <td>{{ '∞' if domain.max_users == -1 else domain.max_users - (domain.users | count)}}</td>
   <td>{{ domain.max_quota_bytes or config['DEFAULT_QUOTA'] | filesizeformat }}</td>
 </tr>
 {% endfor %}
diff --git a/core/admin/mailu/ui/views/aliases.py b/core/admin/mailu/ui/views/aliases.py
index 5e65c86d..bd3da3dc 100644
--- a/core/admin/mailu/ui/views/aliases.py
+++ b/core/admin/mailu/ui/views/aliases.py
@@ -16,7 +16,7 @@ def alias_list(domain_name):
 @access.domain_admin(models.Domain, 'domain_name')
 def alias_create(domain_name):
     domain = models.Domain.query.get(domain_name) or flask.abort(404)
-    if domain.max_aliases and len(domain.aliases) >= domain.max_aliases:
+    if not domain.max_aliases == -1 and len(domain.aliases) >= domain.max_aliases:
         flask.flash('Too many aliases for domain %s' % domain, 'error')
         return flask.redirect(
             flask.url_for('.alias_list', domain_name=domain.name))
diff --git a/core/admin/mailu/ui/views/users.py b/core/admin/mailu/ui/views/users.py
index e3c03848..42eb6c05 100644
--- a/core/admin/mailu/ui/views/users.py
+++ b/core/admin/mailu/ui/views/users.py
@@ -18,7 +18,7 @@ def user_list(domain_name):
 @access.domain_admin(models.Domain, 'domain_name')
 def user_create(domain_name):
     domain = models.Domain.query.get(domain_name) or flask.abort(404)
-    if domain.max_users and len(domain.users) >= domain.max_users:
+    if not domain.max_users == -1 and len(domain.users) >= domain.max_users:
         flask.flash('Too many users for domain %s' % domain, 'error')
         return flask.redirect(
             flask.url_for('.user_list', domain_name=domain.name))
@@ -168,7 +168,7 @@ def user_signup(domain_name=None):
     available_domains = {
         domain.name: domain
         for domain in models.Domain.query.filter_by(signup_enabled=True).all()
-        if not domain.max_users or len(domain.users) < domain.max_users
+        if domain.max_users == -1 or len(domain.users) < domain.max_users
     }
     if not available_domains:
         flask.flash('No domain available for registration')