courier to dovecot migration

[tovises]

I have a quite old home server based on Debian 6.x packages using courier
imap server (exim4, fetchmail, courier, apache2, squirrelmail) to keep my
and my wife emails (about 25G).
I want renew my server to Debian 10 (buster) and got a recommendation to
use dovecot instead of courier. For this purpose I'm using a different PC
as a sandbox. Debian 10 currently offer dovecot version 2.3.4.1
Installation was "peaceful" but stuck with the migration. I have choose
migration using dsync, from Wiki I was choose this command:

For per-user user/passwords use:

doveadm -o imapc_user=foo -o pop3c_user=foo -o imapc_password=bar -o
pop3c_password=bar backup -R -u user at domain imapc:

(foo and bar placed real username and password)

But I get error:
doveadm(tovis at nusi.tovis-lab.mydomain.org): Error: User doesn't exist

Strongly I have no real domain, I'm using "freedns". "nusi" is the local
name of my old server, "tovis-lab" is conjunction with "mydomain.org"
choose from freedns. I was trying short as "nusi" which is in /etc/hosts
file even ip address but the same result. 143 and 943 ports are working
only inside the firewall (OpenWrt).
I was checked using telnet courier on the "remote" box and dovecot on the
localhost. I was able to login.

What should I use as "user at domain" in this situation?

  tovis