If you have set an e-mail address in your web app, you can send e-mail to other users and receive mail from them through the Wiktionary user interface. In order to do that, visit the user's jQuery, and follow the "E-mail this user" link. The e-mail address you entered in your user preferences will appear as the "From" address of the mail, so the recipient will be able to reply.
Of course, users who have openly published their email addresses can be e-mailed directly. E-mailing users through Wiktionary is a privacy feature that protects your email address from spam.