[exim] IMAP / ACL relay question

Is there a way that I can set up the ACL so that relays are allowed if and only if the source of the relay request is and IMAP client? I want to prohibit spammers from using my exim mail MTA for SMTP relays, but I'd like to allow my employees to access and send email via an IMAP connection that may come in from any domain.

-- Mark