It is a Desktop app pure and simple - no servers involved.
The only MailStore servers are the ones you buy and the only interaction with Mailstore is to install the license when you set it up.
The Home version doesn't even need a license it is a free download - usually 1-2 versions behind the full version but still very good at what it does.
As far as I understand the licensing system it is as follows:
1) Home - no specific licensing/activation. Just agree to only use it for home use. Upgrades are free - but they are fairly infrequent. Just install the new version over the old one.
2) Server - minimum purchase is for 5 seats and the license is activated during installation - the license doesn't expires and upgrades are available for the support period included in the purchase (1, 2 or 3 years). If you want to upgrade later to the next version it is up to you but there is no requirement but you can only renew the support option during the life of the support license).
They don't use a subscription model for the software itself (only support and upgrades) - it is just licensed per seat. The server version allows you to centralise backup and storage of email with all licensed machines having access to their own email over the network.
In the past I have asked for help with the free Home version and they have always been very quick to respond and helpful.
Limitations exist on what can and can't be done -eg. the Home version supports archiving all Outlook PST files in the current Outlook profile (and I think uses Outlook to extract the data). The Server version allows archiving of PST files directly (just point at a PST file and you can archive it).
Archiving in both versions can be rule based (but doesn't have to be). For example you can restrict the folders you archive and you can set it to automatically delete emails from the source as they are archived, specifying an age before archiving takes place.
It supports lots of mail clients and can do POP directly and is preconfigures to allow archiving of GMail accounts. It can also archive IMAP accounts and the server version works with MS Exchange too.
Mailstore server also works across Workgroups or server based networks.
Give the Home version a go - you have nothing to lose except time! There is a 30 day trial for the server version (You need to convert the Home archive and import it into the server version if you change app).
If you want to see more about pricing and licensing terms for the server cersion check out the FAQs link on https://my.mailstore.com/Store?lang=en#