So sender's address and recipient's address is readable, also the subject. And all SMTP headers, e.g. the gateway dns name or ip address from where I got the e-mail. So far so good, I cannot deny that I got an e-mail or that I sent an e-mail. I should be also careful on my e-mail subjects.
Also good is that attachment file names are encrypted as well and even on BBC mails the encryption is done per recipient, so you cannot see the other recipients e.g. on the list of used encryption certificates serials.
Recipient Count: 2
Serial Number: 169ad538
Issuer: CN=Verified Email, O=TrustedRoot.org
Serial Number: 9d71
Issuer: CN=StartCom Class 1 Primary Intermediate Client CA, OU=Secure Digita
l Certificate Signing, O=StartCom Ltd., C=IL
CertUtil: -dump command completed successfully.