In Odoo, first, create database on server then install like: sale and account module and mapping with country account like as united state currency is $ then install module:
United States – Accounting install. And If users want to send an email to a customer with an invoice and if the invoice has already been created then the user can attach the invoice in PDF format within the mail and can send email successfully to the customer.

Follow these steps shown below

Step1- First install all modules as per your requirement like as sale and account module and also all account as per your location.

Step2- Then go to the setting and configure outgoing e-mail for your customer like:

Settings -> General Settings -> Outgoing Mail Servers and then create the outgoing mail in database, as per the point given below:

1- In Description test write Gmail.

2- In SMTP Server test write smtp.gmail.com.

3- In Connection Security select the SSL/TLS.

4- In SMTP Port fill is 465.

5- In Username write your Gmail is like as abc@gmail.com

6- In Password write your Gmail password.

Show in screen shot given below