One and ONLY ONE file? Do you know it's filename explicitly? If so, try to do a File System operation on it and that operation will either fail (if the file does not exist) or succeed. If it succeeds, (let's say you did a File Move operation), then set up
a precedence constraint to the Send Mail Task.
Another option would be to set up a Script Task that simply checks for the existance of the file and sets a (Boolean) Control Variable accordingly. Then your Precedence Constraint would be "Success and Expression" and the Expression would be: "@MyControlVariable".
(Note that you don't need anything as complex as "@MyControlVariable == True" because if @MyControlVariable is already of Boolean Type, then just examining its contents will evaluate to either True of False!)
Todd C - MSCTS SQL Server 2005 - Please mark posts as answered where appropriate.