To those less familiar with email standards.. this means the 3rd party service send grid are the ones actually sending mail on their behalf. More specifically,

To those less familiar with email standards.. this means the 3rd party service send grid are the ones actually sending mail on their behalf. More specifically, SPF exists for a domain owner (in this case, popcornnotify.com) to specify who is authorized to send mail on behalf of the host email.popcornnotify.com. This is set in a DNS record and used by spam filters to prevent spoofing and such.