In the SparkPost (an email sending provider) documentation for setting "bounce domains" it says
specified in the [...] mail from header in the SMTP payload
https://www.sparkpost.com/docs/tech-resources/custom-bounce-domain/
But when I set the "MAIL FROM" header I get a response from their server stating
550 5.6.0 Invalid header found (see RFC2822 section 3.6)
I'm using the plugin gomail "gopkg.in/gomail.v2"
What does setting the "MAIL FROM" header actually mean? How do I set it?