How to know the mail is readed or not

There are a few ways to determine if an email has been read or not:

Read Receipt: Some email providers, like Gmail, allow you to request a read receipt from the recipient. This sends a notification to your email account when the recipient opens the email. To request a read receipt in Gmail, go to the email you want to track, click on the three dots at the top right corner, and select "Request read receipt."
Tracking Pixels: Some email marketing tools and services use tracking pixels to track whether an email has been opened or not. These pixels are small images embedded in the email that load when the email is opened. When the pixel loads, it sends a notification to the sender's email account.
Email Client Settings: Some email clients, like Microsoft Outlook, allow you to set up read receipts or tracking for specific emails. To do this, go to the email you want to track, right-click on it, and select "Track" or "Request read receipt."
Third-Party Tools: There are also third-party tools and services that offer email tracking features, such as Yesware, Boomerang, or Streak. These tools can track whether an email has been opened, replied to, or forwarded.
Email Headers: You can also check the email headers to see if the email has been opened or not. Email headers contain information about the email's journey, including whether it was opened or not. To view email headers, you can use a tool like Mailvelope or Email Header Analyzer.

Here's an example of how to check email headers in Gmail:

Open the email you want to check.
Click on the three dots at the top right corner.
Select "Show original."
In the email header, look for the "Received" lines. If the email has been opened, you'll see a "Received" line with a timestamp indicating when it was opened.

Keep in mind that some email providers may not allow read receipts or tracking, and some recipients may not open emails even if they have been sent.