There are mail retriever like getmail4, mpop, mailfilter and fetchmail to retrieve emails from the remote mail servers to local mail box with POP3, IMAP4, ETRN ... protocols. .
we configure getmail4, which is simple, secure, and reliable. Install the getmail with:

Create configuration directory for getmail in home: For more detaills read http://pyropus.ca/software/getmail/configuration.html
For each mail account create a configuraion file getmailrc in .getmail directory: And a bash script to read all these getmailrc files: This is for simple pop mail retrieval and to deliver the mails in the user mbox: The option verbose=1 will print the progress in the terminal and "delete=true" will delete the messages from the server or "delete_after = 10" to delete only mails older than ten days, "read_all= false" to retrieve only new message, "received = false" and "delivered_to = false" prevent getmail from adding additional headers to your mail and and "message_log" will log the actions.
Some mail servers use SSL or TLS like gmail this is with SSL: This to retrieve with IMAP protocol and tell which dirs to reterieve and move on deletion to wich folder:

To deliver the mails to an external MTA like exim4: As you see, we tell getmail that users local email address is user@localhost.
You can use the eache user home mail dirs for destination like qmail delivery: Create three dirs in the Maildir:

Get mail of virtual accounts

When using getmail to retrieve virtual mail accounts, more configuration is needed. Here is an example of a config file used to retrieve mail for a virtual account: