This data is available in the Campaign Management section of Members/Email.
To find this go to Tools > Email Databases > Manage Campaign, select your database and note the OPEN column, this column will show the number of people that have opened an email.
Please Note: Open rates are not an exact science. Standard practice is for the email to include a small image (not visible to the reader) within the email that is downloaded from the email server, when that image is downloaded an "open" is recorded.
This means that an email must be read in HTML format in order for an open to be recorded. Consider the following email clients -
Microsoft Outlook. Outlook is the most commonly used business email client. Later versions of Outlook have been blocking images by default. If an email is viewed without downloading the images, an open will not be recorded.
Also consider the preview pane in Outlook. Whilst a user may well have image download set to automatic, he/she may simply be moving through their email list and click on your email without reading it, this will however record an Open.
Web Email: Web based email systems such as Gmail and Yahoo also operate in a similar fashion to Outlook.
Mobile Devices: Mobile devices such as Blackberry open all email in Plain Text format. Therefore anyone reading your email on this platform would not count as an open.
Finally consider, as Spam Filters become more sophisticated and people are being increasingly overwhelmed by email and therefore being more selective on what they are reading.
Open rates are great for looking at short term trends and will give you a good understanding of whether shifts in your email practices are working (EG moving from a Monday send to a Tuesday send) but should not be treated as gospel.