We currently have a Herald rule which send an email every time a new task is created (currently not working T8464: Herald "subscribe" rules do not take effect immediately, skipping mail about the first transaction).
The email is captured and parsed and some actions are applied to the task.
Like:

• set proper project based on some rules
• set always cc some subscribers (project leaders)
• set task view / edit policy based on project (some users forget to set the correct policy)

Instead of doing this hack (parse mailing list mail) would be nice to have a webhook action.
By having the webhook action we could open to integration into many other 3rd party services.

I don't think I understand. You use the email generated to go back to Phabricator to apply certain changes to the task? Why not do everything within Phabricator?

Becase it's automated and i did not find the feature in Phabricator.

For example:

• if task is created and view policy is not ok then set then automatically update the view / edit policy

I use the Herald rule as a webhook event to a script.

Plus, this is a great feature to open up to 3rd party integrations.
For example:

• i want every time a commit comes in to a special folder a notification in a Hipchat room