What do you hope to find by searching for these particular strings? Open up a message, use ctrl+u to see its source and repeat your search (ctrl+f). That ought to reveal where the string has been found. My guess is it is amongst binary data representing an image, or some identification or message analysis report. General purpose search utilities may not be useful in email files, since many of your messages will be encoded using base 64 or similar and therefore are not plain text.