Assume that you have got your own Postfix/Exim SMTP server connected to the Internet. But you read and write emails on your notebook, that is connected to it just from time to time. How can you flush buffered mail queues when your notebook is connected?
One possibility is to log in and run something like
-f, but by default you have got only several days so and sender will
receive notification emails that his messages still are not delivered
yet. Also you must have secure link (SSH, VPN, etc).
Another possibility is to use POP3/IMAP4 servers, but this is too overcomplicated and bloated for the simple task. Not an option. KISS!
Just tell both of your Postfix/Exim (on the server and notebook) to drop email as a mail via NNCP (nncp-exec) to specified node.
More information for Postfix is here and for Exim is
here. All mail will be stored in NNCP spool,
that after exchanging and tossing will call local
command to deliver them just like that happened on the same machine.