If your server is running PHP 5.3+, you'll get a deprecation notice when ereg() is called on line 351 of wp_mail_smtp.php. This can be fixed by replacing the regex email check with PHP's built-in filter_var() function for checking emails:
// replace the following on lines 351-353
if (preg_match('^[-!#$%&\'*+\\./0-9=?A-Z^_a-z{|}~]+'.'@'.
'[-!#$%&\'*+\\/0-9=?A-Z^_a-z{|}~]+\.'.
'[-!#$%&\'*+\\./0-9=?A-Z^_a-z{|}~]+$', $email))
// change to:
if (filter_var($email, FILTER_VALIDATE_EMAIL))