Hi. AFAIK lines are only wrapped in plain text, not HTML and it works fine for me. So 1 very quick thing to check, in your config you haven't set this to anything higher have you?
$rcmail_config['line_length'] = 72;
Do you have any plugins or customisations or is it just plain 0.5.1? Also please attach the message source from a plain text message you sent from RC, just so we can see the problem in action. Thanks.