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When AcyMailing sends an HTML e-mail to your user, it will include in this e-mail a small picture.
The first time this picture is loaded by the receiver, AcyMailing will increment the number of unique open for that Newsletter and the number of times this user opened it.
If this same picture is loaded again, AcyMailing will only increment the number of times this user opened it so the open rate indicates the percentage of users who opened your Newsletter at least one time.
Some mail clients won't display pictures by default so the receiver will have to click on the link "Always display pictures from..." to display pictures and get recorded as somebody who opened your Newsletter.
So no, the open rate is not 100% accurate and you should only use it to compare results between your Newsletters.
Great > 30% Good – 15-30% Poor < 15%
Technically, it means the statistics picture has been loaded more than one time.
So it could be because the user forwarded to another user or simply because he read it several times.
Some mail software have a "preview" system where the message is loaded when the user access his mailbox, that could cause the stat picture to be loaded a lot of times.