this isn't real promising:
dlg@scotty:~$ telnet lists.roundcube.net 25 Trying 195.159.29.201... Connected to lists.roundcube.net. Escape character is '^]'. 220 mail.maildialog.com ESMTP helo a 250 mail.maildialog.com mail from: dlg@foo.org 250 ok rcpt to: dev@lists.roundcube.net 553 sorry, that domain isn't in my list of allowed rcpthosts (#5.7.1)