Self-hosted Kolab running on CentOS7, hosting e-mail for multiple domains:
$ rpm -qv pykolab
pykolab-0.8.1-2.3.el7.kolab_16.noarch

Client version info:
Qt Version: 5.6.0
KDE Frameworks Version: 5.22.0
Operating System: Linux 4.5.4-1-ARCH x86_64
Distribution: "Arch Linux"
kmail2 5.2.1 (package version 16.04.1-1)
akonadi 5.2.1 (package version 16.04.1-2)
libkolab 1.0.2-2
libkolabxml 1.1.4-1

Problem:
When Akonadi starts syncing the IMAP folders, it seems to download all (or at least a lot of) messages every time, causing a lot of data usage every time. This happens when the Check Mail button is clicked in KMail. I'd expect the messages to be cached on the client side, instead of them being downloaded every time the client syncs. The client is now responsible for somewhere around 2-3Gb of traffic to my server in about an hour, every hour, while I'm logged in (it's a fairly big mailbox).

The akonadi logs repeatedly show these messages for the e-mails:

(17:50:47) kolabobject.cpp(214):        MessageId:  "<be04d323a3efd428473f5cec1de764de@localhost.localdomain>" 
(17:50:47) kolabobject.cpp(215):        Subject:  "[Phabricator] Email Verification" 
(17:50:47) mimeobject.cpp(380): could not find the X-Kolab-Type Header, trying autodetection 
(17:50:47) mimeobject.cpp(93):  Unknown object type:  "text/plain" 
(17:50:47) mimeobject.cpp(93):  Unknown object type:  "text/html" 
(17:50:47) mimeobject.cpp(388): unable to detect object type
(17:50:47) mimeobject.cpp(159): MessageId:  "<E1b9PMN-0004Kc-SR@smtp05.steampowered.com>" 
(17:50:47) mimeobject.cpp(160): Subject:  "An item on your Steam wishlist is on sale!" 
(17:50:47) kolabobject.cpp(317):        no kolab object found  
(17:50:47) kolabobject.cpp(214):        MessageId:  "<E1b9PMN-0004Kc-SR@smtp05.steampowered.com>" 
(17:50:47) kolabobject.cpp(215):        Subject:  "An item on your Steam wishlist is on sale!"

I'm not sure when this started (either server or client upgrade) but I've had this problem for some time now. Clearing the Akonadi cache for the folder doesn't solve the problem.

I checked the Folder configuration through Roundcube for the folder containing some of these messages (my Inbox) and it has set "Mail (Inbox)" as content type. The folder properties in KMail also has "Mail" as content type.

Full text indexing is enabled; I have also checked "Download all messages for offline use" in the client account configuration.

Some possibly relevant imapd.conf settings:

unixhierarchysep: 1
altnamespace: 0
hashimapspool: 1
fulldirhash: 0
lmtp_downcase_rcpt: 1
lmtp_fuzzy_mailbox_match: 1
username_tolower: 1