[Samba] GSSAPI/Kerberos authenticate with Dovecot

[basti]

Hello,

I try to setup Dovecot with Kerberos/GSSAPI and use this howto:
https://wiki.samba.org/index.php/Authenticating_Dovecot_against_Active_Directory#Create_the_Dovecot_user_and_keytab

I also try https://wiki.dovecot.org/Authentication/Kerberos

I can login as windows user on win7 and access shares.
When I open Thunderbird I get the message:

"kerberos/gssapi ticket was not accepted"

For debuging I use Kerbtray.

The Tickets I get are:

MY.FQDN.COM
|-- cifs/dc1.my.fqdn.com
|-- cifs/files.my.fqdn.com
|-- krbtgt/MY.FQDN.COM
|-- krbtgt/MY.FQDN.COM
|-- LDAP/dc1.my.fqdn.com/my.fqdn.com

There is *no* imap ticket.

root at dovecot:~# ktutil
ktutil:  rkt /etc/dovecot/dovecot.keytab
ktutil:  l
slot KVNO Principal
---- ----
---------------------------------------------------------------------
   1    2 imap/dovecot.my.fqdn.com at MY.FQDN.COM
   2    2 imap/dovecot.my.fqdn.com at MY.FQDN.COM
   3    2 imap/dovecot.my.fqdn.com at MY.FQDN.COM
ktutil:  q
root at dovecot:~#

Best Regards,