mailing list template variables; custom message data
Make the mailing list recipient variables available within the sendMessage call so that when sending to a list, dynamic recipient data can be set within the message v:my-custom-data.
So when you create a user within a mailing list you can set custom recipient variables via 'vars' on the create and update mailing list recipient call.
But when you then send a message to the pseudo email address, you cannot tag that message with the dynamic recipient variables within the v:my-custom-data string on the sendMessage call.
If you could access the recipient.vars within the sendMessage call, then you could tag individual messages with custom data that can be digested from the webhooks after sending bulk emails via the mailing list feature. This feature is not currently available, but seems like it should be. Otherwise, we can only digest the granular success of a bulk mail in whole rather than in a user-specific context.