Hi @fu7,
Let me handle this query about SMTP servers for you.
QuickBooks Online (QBO) works by using its email servers whenever you send out emails to your customers and suppliers. Currently, there isn't an option to configure or change the email server QBO uses.
If you'd like to edit, attach files, and email sales forms to your customers within QuickBooks Online, you can view this article: Email and customise sales form.
In case you have other questions in mind, please feel free to drop it in the comments below. I'll get back to you.