Modest is based on Tinymail, and Tinymail is based on Camel. Camel's IMAP support has this feature built-in, but it's switched off in Tinymail.

So I tried patching Modest and Tinymail to switch it on. The result is that it seems to work. However, whenever I try to build the Tinymail libraries from source, the resulting packages cause frequent crashes in Modest. This is with or without my patches, so I don't think it's due to the changes I made.

I tried to get in touch with the developers on the mailing list, but didn't get any reply.

So for what it's worth, here are my patches. Maybe someone else can get the SDK to actually work.

There's still plenty wrong with this. Once an account is selected for synchronisation, it will only download mail once it detects that there are new messages. That usually means you need to go into the folder and look at the message list, then wait for it to finish downloading. And it won't download messages it already knows about. I might, but probably won't, have time to come up with a solution.

Also, it seems that when you're in offline mode and you start modest, it forgets about folders other than the standard ones. That doesn't bother me much, my usual situation is I want to keep reading my news feeds (which arrive in their own folders) even when the train goes underground. But it's not the way it should work.

Anyway:

http://mat.exon.name/tinymail.diff
http://mat.exon.name/modest.diff