The user email page has the following fields:
- From: This is automatically filled in with your Wikivoyage user name.
- To: Again, automatically filled in with the user ID for the user you're sending mail to.
- Subject: A descriptive subject of your email. This will become the Subject: header for the email that's sent.
- Message: The text of the message you want to send. This will be the body of the message sent to the user.
Note that email sent through Wikivoyage is not anonymous. The recipient of the message will see the email address you used to register your user account in the "From" header of the email message. Also, users have the option to disable email delivery to their registered email address. So, there's no guarantee of delivery.
A better option for interpersonal communications on Wikivoyage is editing the user's user talk page. This keeps the discussion out in the open, where other Wikivoyagers can see it and possibly also respond.
Abusive or excessive email through the User email page is considered a grave misuse of Wikivoyage, and is grounds for banning.