If the recipient of the poster via email doesn't actually need the file to be in a Publisher format (to be opened in their version of Publisher), have you considered printing the PUB file poster to PDF via the printer selection menu?



Windows 10 has this PDF print option available or your newer version of Publisher may have an option to Export (File > Export > Create PDF) the poster as a PDF.
Then it's a simple case of emailing the PDF poster for the recipient to view.