Signed file not find in EML!!

hi, I develop a simple program in C# to download mail from pop3 / Imap server.

I Use Chilkat 9.3.1.0 for MS Framework 3.5

When EML email file contains pkcs7-signed attachment file (Content-Type: application/pkcs7-signature; name=conv_stage_PAMIO.pdf.p7m), then this attachment is not found in Chilkat.Email.NumAttachments !!

instead, When EML email file contains pkcs7 attachment file (Content-Type: application/pkcs7; name=conv_stage_PAMIO.pdf.p7m), then this attachment is found in Chilkat.Email.NumAttachments !!

Why this different behavior?

thanks