Returns true if the MDN was signed and the signature verified successfully.
Transmissions that failed at the HTTP layer are never signed correctly. This function is unnecessary when:
a. "Abort on error" is enabled for /as2/send, and
b. "Request signed MDN" option was enabled for the AS2 partner, see Configuring AS2 Partners.
as2-signed(result:AS2 MDN) -> Boolean
A value of type AS2 MDN.