Given on my daily cron should not output any on mongodump, but if there is a  an error I should receive en e-mail. Right now, I do not know if it is really an error or a wrong output stream.

For example:  
mongodump --username backup --password pass --out=test > test.log 2> test.err.log

As you can see, there is no error, but mongodump.log is empty and the attached mongodump.err.log has verbose information and it is not correct.