Good. I asked as I haven't seen my answer flagged with a "like" nor as the "best answer" like is customary here., but no problem!
Anyway I'm a little bit confused: AFAIK you don't need your IP to be in white list if you authenticate with your SMTP server, and I think Horde should do that user authentication, as Roundcube does.
In my server I have that white-list as it is needed by other software (the "mailman" mailing lists manager), but I think there are consequences to that like e.g. opening to every script the possibility to send mail in an authenticated way.
Ok, the problem is not authentication. I don't want open authenticate realy because some users get spam malaware. So I only need to send mail with horde webmail. White list works well. Thank you