Once you have completed the steps to setup Outlook Web Access for S/MIME, and you have previously imported the public keys of recipients you want to contact into Outlook 2010, you can use these steps to send encrypted email in Outlook Web Access.
In Outlook Web Access, click New to start your new e-mail message.
In the new message window, on the top row of icons, there will be an icon of an envelope with a certificate, and an icon of an envelope with a lock. Click both icons.
On the To: line, start typing the email address of your recipient. If you get an auto-complete suggestion for the email address, press delete on your keyboard, so that Outlook Web Access will not use old address information.
Compose the remainder of your email message. Note that the subject line will not be encrypted, only the message body will be encrypted.
Click Send when completed.
If you get a warning that the email could not be encrypted, check to make sure that your recipient is in your Contacts list, and that you have a digital certificate for your contact.