The most common form of email testing is to conduct an A/B split test. This
is a test that involves sending one version of your newsletter to a specified
percentage of your database, whilst sending a modified version to the remainder
of your database. By monitoring the results of each send you determine which
version yielded the desired results. You can, for instance, test variations of your
subject line to determine which is more effective in convincing subscribers to
open your email.