Indeed, currently only a single account in the helpdesk can be configured to send outgoing email at a time. Whichever account is set as the "Default account" will be used. A request to expand this to allow multiple email accounts to be used for outgoing mailing already exists on our customer wishlist, we've added your feedback to this. At this moment in time this feature isn't yet in active development though and we aren't able to provide you with an estimated release date.