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.

Example

If you send an email to record 819, which subject “Are you interested?”, you could set the record id to 819 or a variable, which contain this and create a filter “Subject is equal ‘Are you interested?’ “. If you only want to check the first mail you send, set Sort order to “Created Time ASC” and set the amount of mails to “1”. Then you check, if customer had already opened this mail.