Hi,
you are correct. Due to changes in the Joomla! mailer I think about the Joomla! 3.5 release, there were some problems with sending mails with Visforms, too. In order to make sending mails with Visforms more stable, I changed the format of the Mail From Address and the Mail To address input in the Visforms administration from "text" to "email". As a result both fields are validated for a valid email address (if they are not left empty) and therefore it is no longer possible to add more than one Mail To address. I don't want to go back to the previous state, because it was more error prone. You could try to change a Visforms core file, but your changes would be lost with every Visforms update.
It is the adminstrator/components/com_visforms/models/forms/visform.xml file.
Change type="email" to type="text" in
Code:
<field name="emailto" class="longfield inputbox" type="email" label="COM_VISFORMS_EMAIL_TO"
description="COM_VISFORMS_EMAIL_TO_RESULT_DESC" size="50" maxlength="250"/>
(about line 108)
After that you should be able to enter and store more than on email address. As far as I can see in the code, sending the mails to all these addresses then, should work as in your previous version.
Regards,
Aicha