Yeah, email ids don't go via LDAP. Email ids and aliases are restricted because in other email systems people can use _, - and + as subaddress. Cloudron only supports + right now but might extend it to - and _.
@nebulon IIRC, the _ restriction comes when we had 1-1 mapping between username and email. Maybe it's not relevant anymore. I am more open in allowing it in usernames than mailbox names.