This block could check if an email you send to a record, was already opened. It use the AccessTracker build in with vtigerCRM.
You could create a filter to select one/multiple mails, which will be checked. Only one of the mails needs to be accessed.
The field mails related to this record could be used to check mails, related do another record. If you leave this field blank, the current record will be used. Also you could limit the number of mails, which will be checked. Mails which come later in the sort order, will be ignored.
This result is not very reliable, because lot's of mail clients block images by default. In this case the Opening will not be recognized.