QMail: How to track the activity for a certain email address

I'm rather new to dealing with qmail and Linux, so please be patient with me. I have an issue with a specific email address owner complaining that incoming emails are being received slowly and sometimes, they come though only a day or so after the they have been sent. What I would like to do is to display, say, an SMTP log of the interaction between the emails coming into this problematic email address.

Where should I start looking to begin resolving this issue? I've already run "qmHandle -l" to see what's in the queue, and it notifies me that there are over 90 emails in the remote queue, and about 7 in the local queue.

In short, what I'm trying to do is just view a log of all the traffic between this problematic email address and all incoming emails to this email address.

Many Thanks!! 🙂