Fix E-Mail validation error reporting

src/server/BreCal/schemas/model.py

@@ -8,6 +8,7 @@ from marshmallow_dataclass import dataclass
 from typing import List
 
 import json
+import re
 import datetime
 from BreCal.validators.time_logic import validate_time_is_in_not_too_distant_future
 from BreCal.validators.validation_base_utils import check_if_string_has_special_characters
@@ -506,7 +507,7 @@ class UserSchema(Schema):
 
     @validates("user_email")
     def validate_user_email(self, value):
-        if value is not None and not "@" in value:
+        if value and not re.match(r"[^@]+@[^@]+\.[^@]+", value) in value:
             raise ValidationError({"user_email":f"invalid email address"})