When creating a delivery server of type SparkPost web Api in AppMail, you do it by generating an API key for a SparkPost master account, and for that API key you allow to create webhooks automatically, and this means that AppMail can create the webhooks need for you automatically.
However, when creating a new SparkPost API key for sub-accounts, you don’t have the option to allow that sub-account to also create webhooks, therefore AppMail will not be able to create the webhooks automatically for you.
Up until AppMail version 1.3.8.5, if you were to create a delivery server of type SparkPost web Api in AppMail and the API key belongs to a sub-account, then you would get an error (Forbidden!) and you wouldn’t be able to properly validate the delivery server, so you’d be forced to use a master account.
Starting with AppMail 1.3.8.6, we don’t show an error anymore when the webhook creation fails, that is, so you can also be able to use SparkPost sub-accounts.
This also means that if you are using SparkPost sub-accounts, you have to add the webhook manually in your SparkPost account, like shown in this image: