Microsoft Outlook had problems encrypting this message because the following recipients had missing or invalid certificates, or conflicting or unsupported encryption capabilities.
Secure two-way communication is achieved by both ends having certificates and having both parties give everyone their public key. If this is done then anyone, anywhere can send an encrypted (secret) message to either of these two people. These two people have that same ability and can now send encrypted messages to each other using each other's public key.
This will also work with digital signatures. To ensure someone that receives a digitally signed message can read it, your public key is sent along with your message. And, to ensure that the public key that came with the message is really related to the true sender and not just made up, Sectigo includes their own signature in your certificate to back up your claim.
In order to send an encrypted email to someone Outlook requires that a valid Certificate be associated with the contact who is the intended recipient of the encrypted message. A signed email is the best method of distributing this certificate to those that wish to send you encrypted messages. In order to exchange encrypted messages, both sender and receiver must have a signing and encryption certificate.
To ensure that a certificate has been associated with a contact in Outlook, follow these steps:
A six step process to ensure the presence of the certificate/
You can now send an encrypted e-mail to your recipient.