Mackie, 24 could be playing League One football next season after the Pilgrims were relegated from the Championship along with Peterborough United and will be joined by either Crystal Palace or hopefully Sheffield Wednesday.
National newspaper reports have suggested that the Blades are to make Mackie a £700,000 summer signing - he has also been linked with Derby County and Middlesbrough.
The striker has netted eight goals to date this season for the Home Park club.