Hi Artemiy K., I noticed your profile and would like to offer you my project.
I am planning to rent a dedicated server with a fresh install of Ubuntu [14.04?] that I can install Webmin on. I prefer to use Webmin to manage the server since I have used it before and I don't like to use the terminal if I don't have to. My need from you would be to set up the Postfix [which Webmin will have already installed] so that it is only an SMTP server that three users can use to send email from their laptops with. The users will not login to this new server. The users will just be allowed to relay their email through this server. There is another dedicated server that I use for all the other email, web site stuff and I don't want both send and receive email on the same server since I have been attacked successfully in the past.
You may know that Webmin says that you do not need to use Webmin to configure services on the machine. Webmin just assembles the information about all services on the machine and gives you a GUI to manipulate it. So if you are not familiar with Webmin but understand Postfix I think you can use your own method to set up Postfix.
I hope this is a very easy and straightforward task for you and that you will be interested in spending some time on it.
Thank you for your time.