Use UserIssuerSerialNumber

This policy setting determines whether Outlook uses IssuerSerialNumber as the SignerIdentifier, which enables third-party email client software applications to read encrypted Outlook email messages. For more information about Cryptographic Message Syntax, refer to the RFC 5652 specification. If you enable or do not configure this policy setting, Outlook uses the IssuerSerialNumber as the SignerIdentifier. If you disable this policy setting, Outlook uses SubjectKeyIdentifier for the SignerIdentifier, which might prevent third-party email client software applications from reading encrypted Outlook email messages.