Postie relies on the built-in WordPress Cron process to fetch emails and turn them into posts. However, WordPress requires that there be traffic to your site in order to run the Cron process.

The first check you must do to be sure the WordPress Cron is enabled. Clicking Test Config on the Postie settings page will tell you if Cron is disabled. Look for

Cron: On

If you see Cron: off open your wp-config.php file and find a line like

define('DISABLE_WP_CRON', true);

if present, remove it. WordPress Cron should now work.

External Cron

If you would prefer to have more fine-grained control of how Postie checks for mail or you can’t get the WordPress Cron to work, you can setup a cron job.

From Wikipedia

The software utility Cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals. It typically automates system maintenance or administration—though its general-purpose nature makes it useful for things like connecting to the Internet and downloading email at regular intervals. The name cron comes from the Greek word for time, χρόνος chronos.

The Easy Way

If you are using a hosting company that doesn’t allow you access to cron or you don’t want to mess with it you can use a service like EasyCron, SetCronJob or Beew to access http://<mysite>/wp-cron.php

If your hosting company uses cPanel for managing your domain you can add a cron job through the cPanel interface. See the cron job docs then choose one of the commands listed below.

The Techie Way

Setup a cronjob to access http://<mysite>/wp-cron.php Note that every time you access this page Postie will run – it is like clicking Run Postie on the Admin screen.

Note that before Postie 1.6 the URL was: http://<mysite>/wp-content/plugins/postie/get_mail.php This URL is no longer supported.

Linux

If your site runs on a UNIX/linux server, and you have shell access, you can enable mail checking using cron.

Examples:
This fetches the mail every five minutes with lynx

*/5 * * * * /usr/bin/lynx --source http://<mysite>/wp-cron.php >/dev/null 2>&1

This fetches the mail every ten minutes with wget

*/10 * * * * /usr/bin/wget -O /dev/null http://<mysite>/wp-cron.php >/dev/null 2>&1

Windows

You will need to install wget or curl.
Then use the Task Scheduler control panel to call wget or curl.
Examples:
wget example

wget http://<mysite>/wp-cron.php

curl example

curl http://<mysite>/wp-cron.php

More

WordPress has their own page with additional options: https://developer.wordpress.org/plugins/cron/hooking-wp-cron-into-the-system-task-scheduler/