✅(mailboxes) remove '+' from valid special caracters

We previously accepted '+' as a special caracter during mailbox creation.
We now remove it, as this caracter has a very special meaning and it wouldn't
make sense to create a mail using it.

src/backend/mailbox_manager/models.py

@@ -116,7 +116,7 @@ class Mailbox(BaseModel):
         max_length=150,
         null=False,
         blank=False,
-        validators=[validators.RegexValidator(regex="^[a-zA-Z0-9_.+-]+$")],
+        validators=[validators.RegexValidator(regex="^[a-zA-Z0-9_.-]+$")],
     )
     domain = models.ForeignKey(
         MailDomain,