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A correspondent indicates an "Digital Signature Is Not Valid"

You sent a signed message and the receiver saw that kind of error:

Digital Signature Is Not Valid
This message includes a digital signature, but the signature is invalid.
The signature does not match the message content correctly. The message appears to have been altered after the sender signed it. You should not trust the validty of this message until you verify its contents with the sender.

In most cases, this message is due to an equipment that does not respect RFC and modifies the transiting email content. This modification always takes place on the sender site, usually its anti-virus or anti-spam software is the cause of it.

Of course, it should be checked by an ingeneer but this message is essentially displayed by a standard-non-compliant mail server. In that case, the receiver should ask his supplier a product that respects the email content non-modification standard.

In worst cases, your correspondant won't be able to receive S/MIME signed e-mail before the modication of the faulty element.

To enlighten this issue, send your correspondant an email to both his professional and personal mail box (from an other supplier). He'll then see that in one case it works but not in the other and that the issue is on his side.

Message under Thunderbird: