I found out what makes the folders grey: It the Umlauts in the folder name like “Müller” and the mUTF7 default for folder names in dovecot. The nethserver import script however imported folder names in UTF8.
The UTF-8 folder name are displayed in Webtop but in grey and cannot be accessed. Thunderbird cannot access them either.
Here the difference in the filesystem (the latter “Müller” folder was created in Webtop)
UTF-8: drwx------ 5 vmail vmail 60 Jul 3 23:37 .Outlook-Datendatei.Posteingang.Müller
mUTF7: drwx------ 5 vmail vmail 154 Jul 4 06:28 .INBOX.M&APw-ller
After setting Maildir format to UTF-8 in dovecot.conf everything work. There are three lines to change.
mail_location = maildir:~/Maildir:UTF-8
location = maildir:/var/lib/nethserver/vmail/%%u/Maildir:UTF-8
location = maildir:/var/lib/nethserver/vmail/vmail/Maildir:INDEXPVT=~/Maildir/public:UTF-8
The other possibility would be to rename the files from UTF-8 to mUTF-7 which should be done by the nethserver outlook import script.
I think the preferred way should be to go into the direction UTF-8 for all character encodings, so the Nethserver should support UTF-8 Maildirs (maybe as an option).