how do i limit email loaded into zoho to ONLY leads customers clients potentials?

how do i limit email loaded into zoho to ONLY leads customers clients potentials?

I have over 28,250 emails in zoho and none of the emails I trade with customers can be loaded in due to my account being over full.
90% of those emails are not related to any customer, client, potential, account, they were addressed to me or were addressed to someone not in my lead/customer/client base.

I run a small business and have loads of non-customer related agency and governmental emails to deal with and do not need or want them in Zoho.  How do I limit emails pulled into Zoho to the ones to or from emails loaded into the leads and contacts and accounts I have in Zoho.

This is a serious problem as many of my customer related emails are potentially needed for future legal actions as I deal with life safety issues.