The remote client does not properly flag tampered messages.
The remote host is running the Lotus Notes 5.x email client. Lotus Notes R5 client versions 5.0.5 and earlier could allow a remote attacker to send unsigned email messages as signed email messages, due to a vulnerability in the implementation of the S/MIME standard. Lotus Notes R5 client uses the S/MIME standard to securely send and receive messages with digital certificates. However, Lotus Notes fails to notify the recipient if the email being received has been tampered with. A remote attacker can corrupt signed messages in transit without the recipient being warned of the invalid signature. The message appears to the recipient as an unsigned message.
No known remedy as of August 2002 however at the time of writing Lotus Notes 6.0 is available.