startMessage event return smtp connection instead of custom object

The `startMessage` event returns only `from`, `to` and `connectionId` at this moment. 

I must have access to the `authentication` data which are stored inside SMTP connection object. My mailin server acts differently based on `authentication.username` and `authentication.authenticated` values. 

Because the SMTP `connection` object already have fields (currently mentioned by the API - `from`, `to`, connection `id`) I optimized the code by simply replacing the current custom data object with smtp `connection`.

I hope you will agree with this PR. It really just adds some critical data to the `messageInfo` object. Thanks!