"Posting not allowed" / "Posten nicht erlaubt"

New user here. Trying to get started on GMANE. Reading groups does work,
but posting doesn't. By that I mean that attempting to write an article
typing "w" fails with the message I put in the subject line:

  "Posten nicht erlaubt" - which means:
  "Posting not allowed"

Some Googling has elicited that Tin *insists* on the machine running tin
having a fully qualified domain name, apparently to use that for message
IDs. (Mutt and Slrn do just fine without such a name, though.)

My computer runs Win7; Mutt and Tin and Slrn are all from Cygwin.
The hostname is simply Dago:

MiLu@Dago: ~ > hostname
Dago

I read that I could supply a FQDN in:

MiLu@Dago: ~ > cat /etc/tin/tin.defaults
disable_gnksa_domain_check      = ON
domainname                      = gmx.de
disable_sender                  = OFF

I checked using strace that tin actually opens and reads that file.
However, "Posting not allowed". (The other two settings I tried don't
seem to have any effect.)

Note that this is not some GMANE or auth isssue as posting to GMANE
worked just fine using slrn.

However, being a Vim and Mutt user, I prefer the Tin interface as it's
easier for me to move around and configure. But not being able to post
is obviously a disadvantage. How can I get it to work?

Michael