This is an error message that has caused end user confusion for years. This is normally caused by a user having the default setting to sign emails set.
In Lotus Notes, this is done via the PKI (Public Key Infrastructure) of your existing Lotus Notes ID. Your client signs messages to other Lotus Notes recipients with your public key.
For Internet mail, Lotus Notes keys will not work. You would need to have a X.509 certificate integrated into your Lotus Notes ID. This is not common, as X.509 keys are not generally issued to people who do not need them.
When the client goes to sign the Internet message, it cannot find a valid certificate to use, and generates the error. The way to suppress it is to change the default setting of signing messages. If that is unacceptable, then the person can simply acknowledge the dialog box that appears.
You forgot to say that this error only occurs if you have "Format for messages addressed to Internet addresses" set to "MIME Format" under the Mail tab in your location profile document.
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This was first published in February 2007