@ikerrg,

Treat all CSSU stuff as highly experimental. Stable vs Testing doesn't (in my opinion) mean anything.

My solution for offline IMAP is to use offlineimap to synchronize with my IMAP account (for now, gmail) as well as dovecot as local IMAP server. Modest only connects to this local server (address "127.0.0.1") so that e-mails are always available.

The only thing I yet have to solve is that:
(1) dovecot wakes up the CPU more than it should,
(2) modest doesn't show INBOX unless it detects you are connected. sort kills the idea of offline e-mail.

I use alpine instead of modest. Works perfectly.