Thanks for posting, @Raju001 - welcome to the community! Heads up that I've moved this over into the 
Discussion category.
For app notifications, I’d look at SendGrid, Mailgun, Amazon SES, and SMTPMart, especially if you need reliable SMTP/API delivery. Retool supports both SMTP and SendGrid for automated emails from apps and workflows.
From my experience, SMTPMart is a good option for reliable sending and scaling notification emails, depending on your app’s volume and requirements.