When you add a person in Membership Management, the email address is tested by the server code. This is the same email test that you can run manually via the Membership Tools drop-down menu. The server code does this test when you add someone new to ensure that we do not get a bunch of unusable email addresses added to the system.
This approach is part of what are referred to as "email hygiene practices"... basically, preventing bad email addresses from being sent to and removing old email addresses that consistently no longer work from the system. These are required to prevent as many email system providers will block the server if we keep sending emails to addresses that do not work.
For more information on these types of practices, you can Google the term... see the following, for example:
keap.com/business-success-blog/marketing...what-is-list-hygiene
One practice that we do not *yet* do is to make sure we are maintaining "warm email addresses" (e.g. for Guest and Former Member lists)... I will be implementing that at some point.