Balsa does not use the FQDN for Message-IDs, which means no guarantee, that
an ID exists only once.
I have checked, wether there is any problem on the system (freshly installed
Debian Etch Stable), which could cause this behaviour,
#v+
hajo@Odysseus:~$ hostname
Odysseus
hajo@Odysseus:~$ hostname -d
Zierke.com
hajo@Odysseus:~$ hostname -f
Odysseus.Zierke.com
hajo@Odysseus:~$ dnsdomainname
Zierke.com
hajo@Odysseus:~$ cat /etc/mailname
Zierke.com
#v-
but don't find anything.
There isn't any problem with mail sending and receiving.
Ciao
Hans-Joachim
P.S.: MTA is sendmail.