Hi,
At this time, Express Invoice lets you set/use one email account, no matter if you have multiple business, the email account will be the same for all business profiles. This means that if you are able to send from one business profile, you are also able to send from the other one, as is the same account, using the same settings.
The Expert user suggestion(previous post) is a good practice, by including your email account next to the customers email(separated by a coma "," symbol) before sending, you can have/receive a copy of the invoice to confirm if it was successfully sent or not.
In some occasions, networks and domains could block emails that contain attachments, IF this is your case, as a work around use the option that says "include invoice in the body fo the email" before sending.
If you need further assistance please contact the support staff, they will be more than glad to assist > supportnch@nchsoftware.com