Sometimes, some or all of the users store their mail on separate internal email servers and the Opendium server needs to be set up to forward their mail to those servers rather than delivering it to local mail boxes. This can be configured on a per-group basis in the Redirection subsection of the Mail tab.
Once you have selected a group, you can specify the address of another mail server. Any mail destined for one of the users in that group will be forwarded to the configured mail server by SMTP. These settings will also be inherited by all the descendent groups, unless explicitly overridden. If you need to override the inherited settings, untick the Inherit box of the setting. If you want to turn off mail redirection for a group, just leave the mail server address blank.
Your internal mail server will probably need to be allowed to relay mail through the Opendium system without authenticating. In order to set this up, create a new user group in Users & Groups, then use the Create Network button to add the IP address of your mail server to that group. In Permissions & Limits, select the new group and tick Email clients can relay to external domains and Networks in this group never need to authenticate.
Please also read about the Anonymous group, as this is especially relevant to networks that have multiple email servers.