I have the same problem.
for example, my domain is xyz.com and my smtp is mail.xyz.com
my imap server do not need a domain in the login, e.g. only "user" not "user@xyz.com"
$rcmail_config['default_host'] = 'mail.xyz.com';
$rcmail_config['username_domain'] = '';
$rcmail_config['mail_domain'] = 'xyz.com';
It seems default_host = imap server name
when username_domain = blank
roundcube use usename@default_host =>
user@mail.xyz.com as the email for sending email.
Regards,
Steve