Mailing list software is notorious for changing the textual part of a message and thereby invalidating the signature; however, this problem is not specific to S/MIME, and a digital signature only reveals that the signed content has been changed.
Mailing list software is notorious for changing the textual part of a message and thereby invalidating the signature. This problem is not specific to S/MIME. This is however. The signed content has been changed. This is what a digital signature reveals.