This relay access denied only happen when some of our users send email from their home. They do not have this problem when sending in the office. Could it be their ISP issue? What can we do about this?

Also, when they include a few people in their emails sending from their home, only those email that are hosted in the same zimbra server receive it while external email accounts will return undeliverable message: 554 5.7.1: Relay access denied.

This mean that there is no problem sending email from their home to our office's Zimbra. And since there is no problem sending email from Zimbra to external users, how come there is a problem when sending email from their home to external users thru our office's Zimbra?