That depends on the 3rd party add-on that you installed in Outlook.
The only secure sending features that I know of in Outlook is to add a
digital signature and to encrypt your e-mails. To add a digital signature
means you have to get an e-mail certificate. To send encrypted e-mails
requires that you receive a digitally signed e-mail from the recipient so
you can use their public key to encrypt your message which they then use
their private key to decrypt.