How to send a message or a mail to customer ?

I'd like to ensure we're on the same page, sandeep1.

Are you trying to send invoices to your customers through email? If so, I'll show you the steps. Before doing so, let's set up your sales form message by following these steps:

1. Go to the Gear > Account and settings.
2. Select Sales on the left panel.
3. Scroll down to the Messages section.
4. Add your message under Default email message sent with sales forms.
5. Hit Save and then Done.

I've got this sample screenshot for a visual reference:

You can also read this article to personalize the invoice template: Customise invoices, estimates, and sales receipts in QuickBooks Online.

Once done, here's how to send the invoices:

1. On the left panel, click Sales > Invoices.
2. Put a check mark on the invoices you want to send.
3. Select the Batch actions button and choose Send.
4. Enter your customers email addresses.
5. Hit Send. 

Please see this sample screenshot:

Moving forward, you also have the option to send email reminders to your customers when their invoices are due. Here's an article for more details: Send invoice reminders automatically or manually in QuickBooks Online.

Please let me know if you're referring to something else. I'll be around to provide details and instructions. Thanks!