For every Mailbox configuration group, 1 or more mail servers can be defined. To add a new mail server, open an existing Mailbox configuration and click on the Add button underneath the "Servers" header:
The example above shows how a connection to GMail is configured. The options Move to folder when processed and Move to folder when failed can be used to remain processed emails within the mailbox. Only emails within the Inbox will be processed by the Mailreader.
Note that if you are creating folders within the Inbox, your folder name definitions should include Inbox, for example: Inbox/Processed.