About fastmailusercontent.com

The domain 'fastmailusercontent.com' (and sub-domains) are used by Fastmail to serve untrusted content such as attachments in emails for viewing and downloading. By using a separate domain, we avoid the possibility of Cross Site Scripting (XSS) attacks.

This domain never serves public content, only content for individual users through an authenticated URL structure.

If you have any questions please contact support@fastmail.com