After digging a little bit, it seems it's because I did not set "expunge
= no" in the local repository.
By the way, here is may local repository configuration in offlineimap.
Is this the correct way to do things?
[Repository LocalIMAP]
type = IMAP
preauthtunnel = /usr/local/Cellar/dovecot/2.1.9/libexec/dovecot/imap
holdconnectionopen = yes
expunge = no
Should I rather use maildir storage directly?
Finally, it seems that messages with the Trash flag are not displayed on
my email web interface on the server (it's a Zimbra). Is there a way to
move messages to the trash without setting this flag?
Thanks,
Alan