SMIME signed mail are often proccessed by security mail applicances. Some of them are only proccesing signed mail with a X.509 certificate and the extension ".cer". Notes sents signed mail with a "p7s" attachment. This is RFC5751 compliant. Anyway these signed mails are dropped by some security mail applicances and there ist no possibility to establish a SMIME encrypted connection.
A switch to select the signed internet mail format.