View who connects using hostname

Hello, I am currently in the process of migrating my server to a more powerful one. The hostname will change to something more professional to what I had 10 years ago :). Now I have searched around, and couldn't find anything about this : How can I scan the mail or exim logs to view what email is connecting (imap, smtp) using the hostname instead of the mail.domain.com per example ? The reason for this, before Auto SSL some clients didn't have a dedicated IP + SSL so they connected using the hostname of the server that had ssl. Now that I am moving servers, I would like to see what email accounts is using the hostname and notify them I will eventually push the old hostname to the new server by changing the A record but I wouldn't want any downtime for my clients. Thank you in advance