Hi,
Webmail will not work for SED unfortunately, nothing gets passed to the program for encryption/decryption. If it's standalone that is. If you have a universal server depending on the placement Outlook Web Access will be encrypted, and Symantec .cloud offers a boundary encryption service which would enable web-based email to be encrypted, but if you're referring to hotmail or gmail, the answer is no.
You can send encrypted attachments however, you just drag the file onto the SED program or use the right click encrypt options then just attach that normally