Rather than email invoices to contacts individually as you make them real you can send multiple invoices as a single action from the list of sales invoices.
You must have enabled Office365 integration for your account - follow the instructions for Office 365 in this article. You must also have flagged a contact at every client being invoiced as an invoice cc email address.
Once that is done you simply check the box next to each invoice to be sent and click the email button. Choose the layout to use just as you would for sending a single invoice - the same layout will be used to generate all the invoices you have ticked for selection:
You can edit the body text of the email messages that will be created in the batch send form prior to clicking the Office 365 button to create the draft email messages:
These new messages will appear in the drafts folder of Outlook, ready to send:
Note that if you choose an invoice for any clients that do not have a contact flagged as an invoice cc recipient the following message will be shown when you click the email button on the invoice list:
Click continue to send the invoices to those clients which do have invoice cc recipients set up, or click cancel to abort the whole batch. Flagging a client contact as an invoice cc recipient is done on the main details page of the contact(s):
A client can have several of these recipients, all will be added by default, but you can remove any prior to sending whilst the an email is still in draft form in Outlook.