Add mailhog documentation

docs/best-practice/ourtools.md

@@ -24,3 +24,23 @@ a Google Cloud Storage bucket in a central backup project.
 See [the tool's Developer
 Hub](https://gitlab.developers.cam.ac.uk/uis/devops/tools/gcp-sql-backup) page
 for more information.
+
+## Mailhog
+
+For testing applications which deliver email, we have a
+[Mailhog](https://github.com/mailhog/MailHog) instance
+at `mailhog.devops.uis.cam.ac.uk`.
+
+Mailhog is a mail server that only receives mail sent to it no matter who it is
+sent to or from and makes the mail available to view through via a web fronted.
+The mail is never delivered to it's supposed recipient. This is very useful for
+testing.
+
+The web frontend is protected by raven to only members of the
+[UIS lookup group (ID: 101611)](https://www.lookup.cam.ac.uk/group/uis-members)
+
+Mail can be sent to this from any app for testing by using
+ - Host: `mailhog.devops.uis.cam.ac.uk`
+ - Port: `9025`
+
+The web frontend is available at [https://mailhog.devops.uis.cam.ac.uk](https://mailhog.devops.uis.cam.ac.uk).