diff --git a/core/admin/mailu/ui/forms.py b/core/admin/mailu/ui/forms.py
index 1d27141d..3c476c67 100644
--- a/core/admin/mailu/ui/forms.py
+++ b/core/admin/mailu/ui/forms.py
@@ -76,7 +76,7 @@ class UserForm(flask_wtf.FlaskForm):
 
 
 class UserSignupForm(flask_wtf.FlaskForm):
-    localpart = fields.StringField(_('Email address'))
+    localpart = fields.StringField(_('Email address'), [validators.DataRequired(), validators.Regexp("^[a-zA-Z0-9.!#$%&’*+/=?^_`{|}~-]+$")])
     pw = fields.PasswordField(_('Password'), [validators.DataRequired()])
     pw2 = fields.PasswordField(_('Confirm password'), [validators.EqualTo('pw')])
     submit = fields.SubmitField(_('Sign up'))