Probablemente sea un cambio de versión de PHP o similar en tu servidor.
Recuerdo tener que cambiar un par de líneas de class.phpmailer.php según la versión de php ;)
Edit:
En mi caso, bastó con cambiar una función de la clase, y quedó así:
Código PHP:
public static function ValidateAddress($address) {
if ((defined('PCRE_VERSION')) && (version_compare(PCRE_VERSION, '8.0') >= 0)) {
return preg_match('/^(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){255,})(?!(?>(?1)"?(?>\\\[ -~]|[^"])"?(?1)){65,}@)((?>(?>(?>((?>(?>(?>\x0D\x0A)?[ ])+|(?>[ ]*\x0D\x0A)?[ ]+)?)(\((?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-\'*-\[\]-\x7F]|\\\[\x00-\x7F]|(?3)))*(?2)\)))+(?2))|(?2))?)([!#-\'*+\/-9=?^-~-]+|"(?>(?2)(?>[\x01-\x08\x0B\x0C\x0E-!#-\[\]-\x7F]|\\\[\x00-\x7F]))*(?2)")(?>(?1)\.(?1)(?4))*(?1)@(?!(?1)[a-z0-9-]{64,})(?1)(?>([a-z0-9](?>[a-z0-9-]*[a-z0-9])?)(?>(?1)\.(?!(?1)[a-z0-9-]{64,})(?1)(?5)){0,126}|\[(?:(?>IPv6:(?>([a-f0-9]{1,4})(?>:(?6)){7}|(?!(?:.*[a-f0-9][:\]]){7,})((?6)(?>:(?6)){0,5})?::(?7)?))|(?>(?>IPv6:(?>(?6)(?>:(?6)){5}:|(?!(?:.*[a-f0-9]:){5,})(?8)?::(?>((?6)(?>:(?6)){0,3}):)?))?(25[0-5]|2[0-4][0-9]|1[0-9]{2}|[1-9]?[0-9])(?>\.(?9)){3}))\])(?1)$/isD', $address);
} elseif (function_exists('filter_var')) { //Introduced in PHP 5.2
if(filter_var($address, FILTER_VALIDATE_EMAIL) === FALSE) {
return false;
} else {
return true;
}
} else {
return preg_match('/^(?:[\w\!\#\$\%\&\'\*\+\-\/\=\?\^\`\{\|\}\~]+\.)*[\w\!\#\$\%\&\'\*\+\-\/\=\?\^\`\{\|\}\~]+@(?:(?:(?:[a-zA-Z0-9_](?:[a-zA-Z0-9_\-](?!\.)){0,61}[a-zA-Z0-9_-]?\.)+[a-zA-Z0-9_](?:[a-zA-Z0-9_\-](?!$)){0,61}[a-zA-Z0-9_]?)|(?:\[(?:(?:[01]?\d{1,2}|2[0-4]\d|25[0-5])\.){3}(?:[01]?\d{1,2}|2[0-4]\d|25[0-5])\]))$/', $address);
}
}
Saludos!