In my logs, the endpoint webhooks/mailjet produced a warning:
Received a Mailjet webhook, but no token has been configured. This is unsafe behaviour and will be disallowed in the future.
In /admin/site_settings/category/email?filter=mailjet there is the “Mailjet webhook token” field, with no instructions on how to correctly set it up.
I went to Mailjet and added to the Endpoint URL field: https://discourse.example.org/webhooks/mailjet?t=[your_token]
I replaced [your_token] with one generated by a Token Generator, and entered the same value in the “Mailjet webhook token” field at /admin/site_settings/category/email?filter=mailjet.
That appears to have eliminated the log warnings.