Message Archiving lets VPOP3 keep a copy of all sent/received messages for future reference.
Archived messages can be stored on a local, networked or external hard drive. If the store location is unavailable, then VPOP3 will queue up the messages locally so that they will not be lost.
The archive can be searched on the Search tab using various parameters, and you can copy the found messages back into a user's mailbox.
You can choose to move old messages out of the main archive store into a ZIP file (which can be copied to DVD, external HDD etc) on the Offline tab. If you move messages to ZIP files, then the search can still find the message and tell you which ZIP file is needed to access the message.
Note that the VPOP3 message archive is not the same as, for instance, the Microsoft Outlook archive function. The VPOP3 message archive stores all sent/received messages for later access, for instance in case of disputes. The Microsoft Outlook archive lets you move older messages to another location where they can still be accessed by the user. There is no equivalent to the Microsoft Outlook archive function in VPOP3, because there is no point moving the messages to another location which is still on the mail server; that would not give any benefit to performance, or storage requirements, so would just complicate things for the user, with no benefit to the user. You can still use the 'archive' function of your email client to move messages out of the mail server and into a local message store - that will reduce storage requirements on the server, at the expense of needing to be backed up separately and of not being accessible to all devices or shareable. If you want old messages to be backed up with the server, accessible to all devices and shareable, they have to be stored on the mail server, which means they have to take up storage space on the mail server; there is no way to have them stored on the mail server but not take up space on the server.