|You can change the message priority on an existing message using a filter. To do so open the filters form by pressing F4, press one of the manual filter tabs (1,2,3 or 4 - select one that does not have any other filters in it). Create a filter that searches the "From:" field for an "@" and has an action of "Change Priority" and select the priority you want. Save this filter.|
Select the message(s) you want to change and from the "Tools" menu select "Run filters now..." and then choose the manual filter category that you created the filter in.
If you wanted to select the priority on a message by message basis you would need to write a script.