"Unable to send e-mail. Please contact the site admin, if the problem persists" Drupal error.

Among other possible causes including your hoster's settings you can get this error if your site name (Configuration->Site Information->Site name) contains non-English characters. Specifically because Drupal (as of version 7.14) handles [site:name] token incorrectly when constructing subject field for outgoing emails. You can easily fix this by replacing [site:name] template in email Subject (Configuration->Account Settings->Emails) with [site:url] (if it's entirely in English otherwise delete the token altogether):