Just sell extra storage at reasonable price. That's the most transparent system you can get.
Some users will never hit more than few GBs as it will be near only text. Other people will share 100MB video clips daily or use it as easy way to transfer files betweeen users in company
Maybe have option to expire attachements at separate timer or ability to set a cap where oldest files get removed if it is passed for cost-control-concious companies
Your costs will change and shift over time. Personally, I don't trust anything that says "Free Forever" or "Unlimited". Give a real limit and figure out the transition. "Free now, and no plans to change, but if it does, we will give you one year to transition" is much more confidence building then "Free forever".
Just sell extra storage at reasonable price. That's the most transparent system you can get.
Some users will never hit more than few GBs as it will be near only text. Other people will share 100MB video clips daily or use it as easy way to transfer files betweeen users in company
Maybe have option to expire attachements at separate timer or ability to set a cap where oldest files get removed if it is passed for cost-control-concious companies
Your costs will change and shift over time. Personally, I don't trust anything that says "Free Forever" or "Unlimited". Give a real limit and figure out the transition. "Free now, and no plans to change, but if it does, we will give you one year to transition" is much more confidence building then "Free forever".
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Google was already fairly profitable when Gmail was started, and did a considerable amount of capacity planning.
It was also invite-only for a loooong time.