If you want to send one invoice at a time, just click the Email button instead of the Print button in the Sales Invoicing window.

If you want to email invoices as a batch, then you want to go to Maintain Customers and, for each customer, make sure the Batch Delivery Method on the Sales Info tab is set to Email. Once that has been done, you can batch email invoices by going to the Reports & Forms menu, choose Forms, then Invoice & Packing Slips. The Select a Report of Form window will open. Double click on the invoice format you want to use. In the print window you can set things like a cutoff date (in case you have future dated invoice you aren't ready to send) and a starting invoice number. And you can filter by a range of customer IDs or customer type. I like to click the Refresh List button to make sure I'm getting the invoices I expected. You should review the settings on all three tabs before clicking the Print/Email button.