Thanks for the reply, we have tested this issue on multiple workstation, mainly we have:

Il 2016-07-14 16:02 A.L.E.C ha scritto:

On 07/14/2016 03:08 PM, sgironella@schema31.it wrote:

Hello all,
I've recently posted a bug reporting to the Roundcube community forum
<http://www.roundcubeforum.net/index.php/topic,23529.0.html>
We have noticed a strange behavior with a fresh Roundcube 1.2 install
(no additional plugins enabled nor 'strange' custom config defined).

To reproduce this, send an e-mail using the html editor, leaving a blank
content.

What PHP version? What Mail_Mime version? What OS? Are you using SMTP
server (smtp_server option) in Roundcube config?

Sounds like https://github.com/roundcube/roundcubemail/issues/5283, but
in your case SMTP debug already confirms that we really send malformed
message, so not exactly the same.