I don’t claim to understand IMAP all that well or why various things like this happen. I can’t seem to find any information about this online so I just steer clear of Mail.app and either use the Web-based Google Mail directly or I use Thunderbird (which has the added benefit of return receipts which come in handy). There are a couple features in Mail.app which appeal to me including the data detectors which make it pretty easy to add or update contacts. The downside is this download behaviour consumes a huge amount of bandwidth and in our bandwidth constrained environment, that is a big #fail!
Does anyone else experience this? Any ideas, suggestions? Should I just ditch Mail.app until Apple releases a decent version?