THIS IS THE ERROR MESSAGE, WHEN I TRY TO SEND MAIL-INVOICE in BECKEND ADMIN:

Could not instantiate mail function.
Invoice 1-2012 sent to

Joomla version: 1.5.26
com_invoices_1.0.0.zip


Global Configuration
Mail Settings

Mailer : PHP Mail Function
Mail from: webstran@gmail.com
From Name :wb
Sendmail Path: /usr/sbin/sendmail
SMTP Authentication: No
SMTP Security : NONE
SMTP Port:
SMTP Username:
SMTP Password:
SMTP Host: localhost

INVOICES CONFIG:
Default From E-mail on invoice :webstran@gmail.com
From (email) on emails :webstran@gmail.com

Rgrds.