Complex sentence: 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.
Split into simpler sentences: Mailing list software is notorious for changing the textual part and thereby invalidating the signature. However, this is not specific to S/MIME, and a digital signature only reveals that the signed content has been changed.