Gmail asks for senders to include a header that says "Precedence: Bulk" on any message that is sent to multiple Subscribers.
When this header is present, Gmail will not send autoresponder messages, such as Out of Office notifications, to the sender's address. This prevents your inbox from being flooded with auto-responder messages every time you send an email to your Subscribers.
Don’t worry -- this header is hidden within your email and isn’t seen by your Subscribers.
Message headers are the part of an email that is seen by machines and help facilitate the email delivery process. You can always pull the message headers for any email in your inbox, though, if you're curious! You can find instructions for viewing message headers here.
Gmail doesn’t use this header for any negative actions.
It won’t cause Gmail to place messages into the Promotions tab or Spam folder. It can actually have the opposite effect!
This header lets Gmail know that you are intending to send an email to many Subscribers at once and that they shouldn’t be alarmed -- your email address isn’t being used to send messages to many Subscribers without you knowing.
Otherwise, Gmail might see a bunch of emails are being sent from your domain at once, and be concerned that you weren't intending for this to happen.
The "Precedence: Bulk" header lets them know that you did intend to send an email to multiple subscribers at a time.