how to set up a series of letters?- Welcome
You do that in the welcome messages, either in mailwizz (or in your own CRM).
perfecto> how to set up a series of letters?
login as customer
go to campaigns
choose autoresponder (AR)
> Or my all clients manually added to the mailing list, and configure the answering machine?
If you mean clients as 'clients of using your email service', then they will setup their ARs themselves, or you provide that service to them.
If you mean clients as 'subscribers to the lists in mailwizz', then you create the ARs, and import your existing subscribers and have new ones added by subscription form.
Hope that helps (you can click 'like' for any of the messages if they are useful)