"Gmail" in message-id for altbox.org

I'm trying out a new free web-mail service at www.altbox.org. When I
sent a msg. to myself from there, to there, I noticed that the message
header included "GMail" like so:

Message-Id: < >

Does this mean that altbox.org is using Google's new Gmail service as
a host, or router? I'm thinking that perhaps Google is paying other
mail services to route through them, so that Google can store data
from other e-mail services, in addition to theirs. Am I being too
paranoid here?

Chief Thracian Usenet