Sending signed e-mail messages. An output data stream is created for streaming a signed e-mail message, and streamed attachment data is read. In response to receiving a portion of the read streamed attachment data, the received portion of the attachment data is digested to generate a digest value, and the received portion of the attachment data is sent to a mail server via the output data stream. The received portion of the attachment data is smaller than the size of the attachment data. The digest value is updated as additional portions of the streamed attachment data are received and digested. In response to sending all attachment data to the mail server, a signer generates the signature data by signing the digest value using a signer's private key, and the generated signature data is sent to the mail server via the output stream.