Has the vmail folder correct rights and owner vmail?
[root@server2 ~]# ls -l /var/lib/nethserver
drwx------ 8 vmail vmail 152 Mar 25 21:44 vmail
You may send a mail from one local user to another with roundcube and check if it works and post the relevant part of
Please provide your installed packages to check your mail server implementation and if somethings missing…
rpm -qa nethserver-*
Seems like firstname.lastname@example.org logged in with wrong credentials.