Encrypts (and optionally signs) the content of the mime_part and then replaces the content with the new, encrypted, content.
true if the content should also be signed; otherwise, false
the key id (or email address) to use when signing (assuming sign is true)
a set of #GMimeEncryptFlags
an array of recipient key ids and/or email addresses
true on success or false on error.
ErrorWrap
Encrypts (and optionally signs) the content of the mime_part and then replaces the content with the new, encrypted, content.