In the reports from your newsletter sendings you can see statistics on how many email messages that have been opened.
Unique opens - Shows the number of subscribers that have opened the email. If a subscriber has opened the email more than once, only the first opening will be counted.
Total opens - Shows the total number of times the email has been opened. If a subscriber has opened the email more than once, all opens will be counted.
An opened email is an email that has been opened in its graphical (HTML) format.
Emails that have been opened in the text version or where the recipient has chosen not to download the images are not counted as opened emails, unless any links has been clicked in the email - then these emails will also be counted as opened.