The Edit User → Prune Rules tab can be used to create/edit rules which cause messages to be automatically deleted according to specified criteria.

To add or edit a “prune rule”, press the Add Rule button. This will add a new rule to the table, with a set of default settings. Don't worry, this rule doesn't take effect until you press the Submit button.

Now, you can simply alter the settings in the columns in the table, by clicking them. The settings define which messages should be deleted, not what should happen to messages.

You can edit prune rules which apply to all users by going to Settings → Database → Message Store, and using the Global Prune Rules section at the bottom - the rules have the same configuration as the user prune rules.

The folder column defines which folder(s) the prune rule applies to. Initially this is set to Inbox, but you can click on the folder name to set it to any folder name you wish. Alternatively, you can set it to * to match all the folders for this user (note that you cannot use wildcards in conjunction with other characters (eg “Inbox/*” will not work as you expect).

The age column defines how old messages are before they will be deleted. This is set in days, and is based from when VPOP3 first saw the message. It is not based on when the message was written or sent. If the Age is set to 365, then VPOP3 will automatically delete any messages which were received by VPOP3 over 365 days ago.

The size column defines how big messages should be before they are deleted. This can be useful to only delete big messages, or to delete big messages sooner than smaller messages. The default is for no minimum size (all sized messages), but you can set it by clicking in that column.

The read column indicates whether this rule will apply to read messages, unread messages or either. This could be useful if you only want to delete messages that have been marked as read.

The flagged column indicates whether this rule will apply to flagged (starred) messages, unflagged messages, or either. This could be useful if you want users to be able to 'star' important messages so they don't get deleted automatically.

If the deleted column is checked, then VPOP3 will only apply this rule to messages which have the IMAP4 'Deleted' flag set. If this column is not checked, then the rule will apply to all messages. This can be useful if your users do not purge (or 'expunge') folders regularly

If the spam column is checked, then VPOP3 will apply this rule to messages marked as spam by VPOP3. This can be useful if you are not using the VPOP3 quarantine facility, but are, instead, having VPOP3 deliver the messages to the user, and are using VPOP3 Message Rules or email client rules to put spam messages into an IMAP4 folder.

Notes:

• the 'age' setting applies to when the message was received by VPOP3, not when any other flags on the message were set. So, if you have the rule set to delete read messages over 365 days old, and you have an unread message which is older than that, it will stay there, but as soon as you mark the message as read, it could be deleted immediately.
• the prune process takes place periodically in the background, depending on the number of messages needing pruning. If VPOP3 checks for messages to prune and doesn't find any, then it can wait quite a while before checking again, but if it finds that lots of messages need pruning, it will check more frequently. There is no way to tell when messages will be deleted automatically.
• Messages that have been pruned can still be recovered using the VPOP3 Recycle Bin feature until they expire from there.