Fastmail bots

As part of providing the service, Fastmail may connect to an external server on behalf of our users. We identify ourselves when we do so in the User-Agent header field sent with the HTTP request. We use our internal user agent for two main purposes:

Image proxying

When a user opens an email with links to embedded images, we load and proxy those images via our servers on the user’s behalf. This enhances user privacy by ensuring that there are no browser cookies in the request, that their user agent string is obscured, and that their source IP address is hidden.

To achieve maximum compatibility, the user agent for this bot appears (mostly) as a common browser user agent string, with “FastmailUA/1.0” appended. The full user agent string we use is:

Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/118.0.5993.117 Safari/537.36 FastmailUA/1.0

Calendar fetching/syncing

When a user subscribes to ICS feed calendars, or sync to/from remote CalDAV calendars, we have to fetch/store those calendars on a regular basis to sync the content with the user’s Fastmail calendars.

Our requests for the calendar data use a basic user agent string:

FastmailUA/1.0

Detecting the Fastmail user agent

For a generic way to detect a Fastmail related user agent, we will ensure that all user agent strings always contain the string “FastmailUA”.