The option of sending email invitations via the 1KA system enables tracking of respondents. Thus you can check who from the list responded and who did not, which provides a basis for sending reminders. This functionality is available to users who have bought a 3KA package combined with their own SMTP settings.
Tracking is disabled if the survey links are sent through your personal email account and the URL is identical for all respondents (thus the survey is truly anonymous). In this case, it is not possible to distinguish who completed the survey and who did not. However, previous experience has shown that the underlying problems outweigh the potential benefits brought by such complete anonymity.