Evolution (or KMail) incorrectly misidentifies email as not valid signed (or signed)
I am not sure which program is messing things up to be honest, since I am not certain what exactly is included in the signature. It seems that it might be due to line endings.
I have attached the offending mbox file. When loaded into KMail it will be shown as valid, when loaded into evolution it will be shown as invalid signature.
I will add a link to the an equivleent bug report to KMail (as soon as it is opened).