If you don't have a signing certificate installed for the account from which you are sending the email, though, when you click on Send, you will see the message "You must first install a valid signing certificate before you can send a digitally signed message. Do you want to send this message without a digital signature?"
If you don't have an encryption certificate installed for the account from which you are sending, you will see the message "You have not specified an encryption certificate for this account. Once you send this encrypted message, you will not be able to read it. Do you still want to send this encrypted message?"
For the above message, you can click on Continue to send the email in encrypted form to the recipients, but you won't be able to read the contents of the message when it appears in your Sent folder.
To add an encryption and/or signing certificate to the account, take the following steps:
If your system doesn't have a public key certificate for a recipient to encrypt the email for that recipient and you are using an account on a Microsoft Exchange Server for your Outlook account, when you click on Send, you will see the message "Encryption certificates for some recipients could not be found in the Outlook Contacts. Do you want to search the Active Directory servers for them?"
If you click on OK, a search will be performed on the Active Directory (AD) servers. If an appropriate certificate is found then the encrypted email will be sent to the recipient.