TMail21 is built for teams. Registering with a work email is the easiest way to start or join a TMail Organization. When a work email is used, the domain name associated with the work email is automatically extracted from the work email and associated with the TMail Organization. Also, anyone with a work email with the same domain can auto-join the team. This makes the onboarding experience significantly easier.
For example if your work email address is firstname.lastname@example.org, then the acme.com domain will automatically be associated with the Organization. Also your TMail address will look something like joe$acme.com instead of joe$acme.tmail21.com.
Note that even if a work email is used, the administrator can still add users whose email domain does not match the associated domain name. Its just that they cannot auto-join.
Using a Personal EMail Address
A personal email address is appropriate for creating a personal TMail account. You can also create an Organization TMail Account with a personal email address. However, there will be no domain associated with the Organization and users will not be able to auto-join based on that domain. A domain name can be later associated with the Organization. However, this will likely require some I.T. support.