I'm using Zimbra server as a backup server with fetchmail configurations and main server is sitting in the cloud. Incoming mails are fetched through fetchmail and forward it to local server. And outgoing internal emails are delivered locally.

I've some employees who works outside. They currently using main server to send and receive emails. The problem I face is when internal user abc.company.com send an email to xyz.company.com. If xyz works outside he won't be able to receive the mail. Is there any solutions for this?