the transport requirements of RFC 2821, do not accept messages that have
more than 1000 characters per line, including the CRLF. Therefore, mail
applications must not create such messages. To work around this issue, turn
off the ESMTP verb (or chunking) capability on the Exchange Server computer
and force the Exchange Server computers to format messages in typical SMTP
format when the message is being relayed."
To do this, see http://support.microsoft.com/kb/257569/