As of now the Helpdesk plugin can only send mail from a predefined emailaddress in the settings. For bulk issues this is fine.
However, every redmine user has a personal emailaddress (within redmine profile).
Can the below be created?

For each reply I can select “Send from my personal e-mail” and then RedmineUP Helpdesk will use the emailaddress that is defined in the current user profile to send the email out and override the default send from address.
If using a email relay service like postmark for outgoing email with domain validation spam alerts will not be a problem.