According to Google Mail guidelines, bulk mail must contain the header
Precedence: bulk
What does it do? I could not find a RFC describing the effect on mail delivery.
Some background: I'm working on scripts that will send 500k+ mails daily. They are different kind of messages: account related mail (delivery critical), but also notifications of new content (delivery not critical).
The exact meaning of Precedence: isn't standardised, but it prevents some mail servers from sending vacation and bounce messages, and may be used by service providers to deprioritise bulk mail during busy times so that "personal" mail continues to be delivered quickly.
From RFC2076:
Non-standard, controversial, discouraged. Sometimes used as a priority value which can influence transmission speed and delivery. Common values are "bulk" and "first-class". Other uses is to control automatic replies and to control return-of-content facilities, and to stop mailing list loops.