SMTP Error (550): Failed to add recipient "XXX" (Please turn on SMTP
Authentication in your mail client.
XXX (XXX.local) [XXX]:XXX
is not permitted to relay through this server without authentication.).

You need to set $config['smtp_user']='%u' and $config['smtp_pass']='%p'

See https://github.com/roundcube/roundcubemail/wiki/Configuration#sending-messages-via-smtp

Best,
Jan