[Plugin: BackUpWordPress] error in activation

i got this error when trying to activate backupwordpress plugin..how to fix?

Warning: main(/home/…/domains/domain.com/public_html/main/wp-includes/class-phpmailer.php) [function.main]: failed to open stream: No such file or directory in /home/…/domains/domain.com/public_html/main/wp-content/plugins/backupwordpress/bkpwp-classes/manage_backups.php on line 1052

Fatal error: main() [function.require]: Failed opening required ‘/home/…/domains/domain.com/public_html/main/wp-includes/class-phpmailer.php’ (include_path=’.:/usr/local/lib/php’) in /home/…/domains/domain.com/public_html/main/wp-content/plugins/backupwordpress/bkpwp-classes/manage_backups.php on line 1052