it works! thanks!
just to wrap that up for anyone interested, the code below modifies the url of .eml files (in a quick & dirty way) so that they open in the default OS mail app:
on performSmartRule(theRecords)
tell application id "DNtp"
repeat with theRecord in theRecords
set myURL to URL of theRecord as string
set mystring to text item 2 of my splittext(myURL, "%3C") as string
set mystring to "message:%3C" & mystring
set URL of theRecord to mystring
end repeat
end tell
end performSmartRule
on splittext(theText, theDelimiter)
set AppleScript's text item delimiters to theDelimiter
set theTextItems to every text item of theText
set AppleScript's text item delimiters to ""
return theTextItems
end splittext