How does double tick feature work when your email gets open?
There are three variants of the check-mark icon that you usually see in your inbox.
Grey Double Tick
Grey Double tick denotes that recipient has not yet received that email. This might happen due to any network issue on the recipient's side.
Single Blue Tick with a Grey Tick
Single blue tick with a grey tick denotes that the recipient has received the email but yet open the email.
Blue Double Tick
Blue double tick denotes that the recipient has opened the email.
Exclamation mark denotes the mail is bounced. In this case, please verify the email address of the recipient.
Refer to the GIF below to understand this process practically: