Postbox does not, and will not, support email tracking (as it is against our company ethos) but you can use a 3rd party service such as Bananatag to track message opens.
There are two steps to getting this set up:
1) Set up a new SMTP server in Preferences | Options > Accounts > Outgoing Servers, then enter the Bananatag settings for that server:
Connection Security: SSL/TLS
User Name: your Bananatag user name
2) Create a second "Identity" for the account you wish to use in Preferences > Accounts > [your account] > Identity, then select the new Bananatag SMTP server for that identity.
You may need to change your display name in the new identity to something recognizable such as "G. Washington" so that you can differentiate the normal identity from the Bananatag identity. Note that this display name will be sent with your messages.
3) When composing simply switch between the identities when sending, depending on whether or not you want to send the tracking image or not.
Note, that this feature is dependent on recipients loading the tracking image, which they don't always do.