Syntax error in parameters or arguments
This is similar to Error # 500, although this is specific to the additional 'parameters' or options that are being used.
This is most commonly an issued with an incorrect/invalid email address, however can also be due to anti-spam filters as referenced in the similar Error # 500. If you've confirmed a correct email address, I suggest reviewing the fixes for Error # 500.
Remote host said: 501 Invalid Address