Right now, the dropdown where you can define the sender of an email offers you the options "System email", "User email" and "Custom". If the Confluence user account has an email domain different from the default Confluence mail server, the option "User email" is disabled and cannot be selected. In this case "System email" is preselected, else we use "User email" as default. It might make sense to add a configuration option that controls if "User email" is offered as default in the cases where it's assumed to be possible, because some customers likely will always want to use the system email for sending.
