One sane way to test anything is based on requirements, which you probably don't have, unless you want to extract them from RFC's. Here's a number of tables of

One sane way to test anything is based on requirements, which you probably don't have, unless you want to extract them from RFC's. Here's a number of tables of standards, UI's and features of email clients: https://en.wikipedia.org/wiki/Comparison_of_email_clients https://en.wikipedia.org/wiki/Comparison_of_email_clients You could study those tables, think about it for awhile, and come up with a subset of derived requirements, and test against that.