This will create a tag in email headers that can be used for filtering by receiving servers. I use this image for relaying through exchange online, and the nature of how I run my containers prevents me from setting static IPs as a filter. With a tag for exchange to look at, I can make sure emails from the relay are not hitting the junk folder.
Without setting the HEADER_TAG variable, the script will create a randomly generated tag and move on. It is not a requirement to make use of this feature.
