String toAddresses = "address1@address.com; address2@address.com";

// Add to To: header
  message.addRecipients(Message.RecipientType.TO, toAddresses);
  String ccAddresses = "cc_address1@address.com; cc_address2@address.com";

// Add to Cc: header
  message.setRecipients(Message.RecipientType.CC, ccAddresses);
  String bccAddresses = "bcc_address1@address.com; bcc_address2@address.com";

// Add to Bcc: header
  message.setRecipients(Message.RecipientType.BCC, bccAddresses);