Helpdesk app in your Webasyst account offers you two basic methods for establishing communication with your clients.
Method A. The most obvious option is to send back your replies as email messages. Helpdesk adds the request ID to the subject of such emails, which looks like [ID: 12345-987654]. Should a client desire to request additional information from you, he can simply reply to the your message. Helpdesk will recognize client's reply by the ID in its subject and will add it to the corresponding request's history instead of creating a new request. To ensure that this happens exactly the way you want it, add appopriate rules for such "replies to replies" in your email source settings, which will auto-execute selected actions for requests in different states, when a reply-to-reply is received.
You may have different reply-handling rules for each of your email sources.
Note that not only your clients can send you replies to notifications received from your Helpdesk. Your colleagues can also reply to them. For example, if you forward a payment-related request to your financial manager, then a manager's reply sent from their email client will also be added to the discussion, as if an ordinary action were executed in Helpdesk. It is convenient, but this approach has some drawbacks. For example, if everyone will be replying from their email clients, then the cited original messages will be appended to each reply and thus will occupy excessive space on the request-viewing page and in your database.
Method B. The recommended option is to display your replies to support emails only in the Customer Portal rather than to send them in return emails. To inform a client that there is a reply, you can simply send him a notification from an auxiliary address like firstname.lastname@example.org, with a link to the Customer Portal, without including your reply in that notification. In this case a client will have to log into their online account to view your reply and to write to you again, if necessary. This method makes clients' messages "clean" as they do not include unnecessary citation, and you can instill some discipline in your clients to make your cooperation more efficient.