I know very little about how this works, but what I recall reading someplace is that the retrieval limit is just for the messages themselves. It still retrieves all of the hundreds/thousands of headers, but only the full message of the last 20. I found this incredibly frustrating as well, and it creates a real mess. I think I ended up having to run some code in xterm to delete the user account folder because even when uninstalling and reinstalling, it retained some of this information.

I was doing all this a while ago, so it is possible that things have been resolved in newer versions, but based on your post, it seems like the main problem I was having (the thousands of msgs making Modest unusable) is still there.

There have probably been a bunch of discussions about this issue on this forum, so I suggest that you search for "Modest" and see what you find.