Gmail feedback loops

Mails that are sent through SMTPeter now automatically get an extra "feedback-id" header. This header allows us to receive feedback from Gmail.

The "feedback-id" holds a number of numeric identifiers, for example your customer and account ID's. When Gmail notices that a lot of spam is sent from our servers, it mentions these identifiers to us, so that we can take action and block specific senders.

This is beneficial for legitimate senders, because we can block and remove accounts that are using SMTPeter for the wrong reasons. This improves the deliverability of the other senders.