Roundcube : ''Message have been sent'' , but nothing happens

How do I debug ?

 

@till @Taleman @nhybgtvfr
Thank you so much for your support. I have solved the issues
For people that was having the same problem ( maybe helpful for someone on the future )
1) I'm using VPS , witch mean I can't follow the the guide @Taleman linked me https://www.howtoforge.com/tutorial/ubuntu-lts-minimal-server/2/#g0.0.7 . But no worries , just proceed with the point number 8 ( apparently need to be done before installing the ISPConfig , otherwise is not gonna work )
2)Proceed with https://www.howtoforge.com/tutorial...l-pureftpd-bind-postfix-doveot-and-ispconfig/
3)Done