Support page says that "Posts from non-subscribed senders will be checked by the moderator first."

[sikkerhedorg]

On the page ​http://roundcube.net/support the following is stated:

Posts from non-subscribed senders will be checked by the moderator first.

In fact, they will simply be rejected. Example of such a rejection is seen below:

This is the mail system at host mail.sikkerhed.org.

I'm sorry to have to inform you that your message could not
be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can
delete your own text from the attached returned message.

                   The mail system

<dev@lists.roundcube.net>: host gigo.com[216.218.228.114] said: 550 5.7.1
    <dev@lists.roundcube.net>: Recipient address rejected: Please wait 5
    minutes after you subscribe before posting to dev@lists.roundcube.net; or
    subscribe/resubscribe at http://lists.roundcube.net (in reply to RCPT TO
    command)