The format and detail provided by a specification varies from project to project. The specification for your project may look very different, but may still have these basic elements in common.
Many people have email accounts on various servers.
Some email clients do not handle multiple accounts well. Private and public email portals and gateways usually allow accounts only on their own servers. Authorization must not require an account on the application's server. Pages that require account information must not be displayed to unauthorized users. To reach the widest audience, the application should be able in multiple languages.