Currently it is not possible to call up an email from a sticky note in Notezilla for Windows.
If you wish to call up a file from a sticky note then you would need to add the file link to the sticky note. Use insert a hyperlink using the right click menu (Standard editor) or Markdown toolbar (Markdown editor) and paste the file path. Also if you drag and drop the file into the sticky note, it will create a hyperlink inside the note so that you can easily open the document.
Once you have done this, you can open the file from the sticky note using the hyperlink inserted.
This short video will give you more clarity on this: