In Taxi we group together emails by projects and mailings. A project might be "weekly newsletter" and a mailing might be "week 3". You can make multiple versions of a mailing, for example Male or Female, English or French.
You can make multiple versions of an email campaign with one click. And as additional versions always inherit content from the master version, you can still update content once and have it apply to all versions.
Sometimes you dont need to let everyone access everything. In Taxi you can restrict access to given Projects, Mailings or Versions, as you need.