You're welcome
A groupware is a software solution desgined to help multiple people work towards the same goal. https://en.wikipedia.org/wiki/Collaborative_software
So by definition a mail system is a groupware, but there are multiple other kinds of groupware, like owncloud or skype, ect...
What you are refering to is a typical user mail solution so you should call it a "mail solution" instead of "groupware" to avoid ambiguity
In my previous message i asked you about the "why" of you wanting to use a groupware, and you still have not answered this so i cannot still help you further
In other words:
- who will be using your mail server?
- in which context: business, association, family, ...?
- how many people?
- who will maintain the server in working condition (install, updates, backup)?
- what hardware will it run on server side?
- where the server will be hosted? How fast will be the bandwidth?
- what hardware will it run on client side (PC, mac, iphones, android phones, ...)?
- give as much other details as possible
Also when you say "Collecting mails from different POP3 accounts and delivering them to user mailboxes (fetchmail / getmail ?)", do you mean that you want users to receive mails from other mail services to your mail service? Can you give more details about that as well?
Last but not least, about the solutions you described:
- iRedMail: sounds good
- Kolab: might be good from what i've checked but can't find support forum
- SOGo 4: sogo is not a full mail solution, it's a gateway between a mail server (backend) and the users (frontend). It's integrated in iredmail.
Zentyal: it's designed to be a all-in-one server solution including mail, which makes it imho not a right choice for what you're looking for.