I am fairly new to SME 7.5.1 and would like to know how I can search a log file for a rejected message or sender. For instance I have staff users experiencing lost emails from other ".k12.il.us" domains.
There's no guarantee that your server will have any log messages related to the missing emails. If those domains have faulty mail servers or ISP's with faulty mail servers, your server will never have any contact. If the DNS for your domain is not properly set up and reliable, remote mail servers may never try to deliver messages to your server, or will contact some other server.
All incoming email transactions are logged in /var/log/qpsmtpd/xxxx files - with /var/log/qpsmtpd/current the most recent file.