There are two possibilities. Either it's "qmail" or "gmail."
1] qmail:-
qmail is a modern SMTP server which makes sendmail obsolete, written by Dan Bernstein, who also has a web page for qmail. qmail is a secure package. You can download netqmail 1.05 (Redhat RPMs, and Debian .debs, HP-UX, Gentoo, and OpenBSD ports) and redistribute qmail for free. You can get the "big picture" of how qmail is organized. You should read Life with qmail
A number of large Internet sites are using qmail: USA.net's outgoing email, Address.com, Rediffmail.com, Colonize.com, Yahoo! mail, Network Solutions, Verio, MessageLabs (searching 100M emails/week for malware), listserv.acsu.buffalo.edu (a big listserv hub, using qmail since 1996), Ohio State (biggest US University), Yahoo! Groups, Listbot, USWest.net (Western US ISP), Telenordia, gmx.de (German ISP), NetZero (free ISP), Critical Path (email outsourcing service w/ 15M mailboxes), PayPal/Confinity, Hypermart.net, Casema, Pair Networks, Topica, MyNet.com.tr, FSmail.net, Mycom.com, and vuurwerk.nl.
Charles Cazabon, Dave Sill, Henning Brauer, Peter Samuel, and Russell Nelson have put together a netqmail-1.05 distribution of qmail. It is comprised of qmail-1.03 plus the recommended patches, some documentation, and a shell script which prepares the files for compilation.
2]gmail:-
gmail is email service provider which is given by the moderators of google.
However gmail is in progress and we are usin g it's beta version but soon after the completion of gmail we'll get final version of gmail.
Hope that you will understand.