The "X-" in the mime type is the mistake and not the "micalg". I pass this on to the s/mime plugin programmers. By the way, according to my tests, only the Thunderbird is so correct to exactly request the MIME type. Other tested email clients, including IBM Notes, apparently compensate for this. So I wrote to the wrong developer team for the bug and apologize for my audacity due to a translation error.
For everyone [https://www.iana.org/assignments/media-types/application/pkcs7-signature here] again is the mistake: Content-Type: multipart/signed; protocol="application/'''x-'''pkcs7-signature"; boundary="--=_V1R2=_=q1202.27865.7237_1" MIME-Version: 1.0 ... ----=_V1R2=_=q1202.27865.7237_1 Content-Type: application/'''x-'''pkcs7-signature; name="smime.p7s" Thanks to everyone who want help me. You can close the Ticket. ------------------------------------------------------ https://bugzilla.mozilla.org/show_bug.cgi?id=1405234 https://support.mozilla.org/en-US/questions/1282926
