The Mail Retrieval app (fetchmail) can conveniently retrieve mail from other servers allowing the centralization of e-mail on a single ClearOS server.
If your system does not have this app available, you can install it via the Marketplace.
You can find this feature in the menu system at the following location:
Any number of servers can be added to the mail retrieval list by adding entries. The interval polling time can be configured from 5 minutes up to 3 hours.
Have a look at the system logs if you are having problems. The mail retrieval system (fetchmail) logs information to /var/log/maillog. Ignore any entries you see similar to:
Server CommonName mismatch: localhost.localdomain != mail.clearfoundation.com