Subject: Your Dropbox needs more space!
Send this type of email to encourage upgrades to paid or higher plans at the most opportune time, as people actively approach any limitations on usage.
Similar to the trial-related upgrade emails, be clear about the status of the account. You can personalize the email to feel more relevant or urgent by including actual usage details.
Don’t spend too much time in this email on making a persuasive pitch. In this situation, customers are active enough to reach the next tier, so they already realize the value of your product or service. What would be most useful to them at this point is to explain any new benefits of upgrading.