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Gmail and Adsense make sense. You are probably still trying to test the strategy of giving away so much free space, the actual percentage of space used in average by user, and if you can make a profit from such endeavor solely on advertising income. To make things more exciting, give us *Gmail for your Domain tied to Adsense* and you might increase that likelihood of profit with Gmail by reducing cost per user and increasing advertising income.
Webmasters will encourage their users to sign up for an email account, which will open up a new diversity of channels for users. Advertisers can then have the option to "Advertise on this Site" and further increase income, advertiser base, user base, and traffic for your company and for Webmasters.
Furthermore, as you keep increasing Gmail's user base through third party domains, you will decease your storage costs, due to the fact that you will be able to store email between Gmail users only once. In this case, all email between ABC.com, XYZ.com, and Gmail.com, in the event that ABC.com and XYZ.com run Gmail for your Domain, Gmail will only have to store email messages between such only once, reducing the cost per user.
What do you guys think?
The problem for Google with this would be one of control. It can assure its sponsors that its ads are showing on respectable pages because it can monitor the quality. It can't do the same through email where someone may be sending/receiving pirated MP3s or port attachments (or the very nature of the email message itself).
To be fair, this is the same problem with Gmail itself too. But it's the only reason why I see Google passing on this logical next step to monetize its publisher relationships.