What do you mean by securing?
In general: the ASA has no application-layer inspection for IMAP. To increase the security you could deploy a scenario like the following:
1) Configure your clients for IMAPs.
2) On the ASA configure a port-forwarding for TCP/993 (which is used by IMAPs) to a DMZ-server running an IMAP-proxy. This proxy could be NGINX or dovecot (there are probably more imap-proxies available)
3) On the proxy configure forwarding the traffic to the internal Exchange server which again has to be allowed on the ASA.
With this setup you don't have to expose your exchange directly to the internet.
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