Although i feel pretty dumb that i had to waste so much time to realise this.
I hope it will be of help to someone else
.
I noticed that there two details to keep in mind.
1. Password must contain everything, chars (capital and not) , numbers , symbols and most important maximum 12 chars length
2. When you insert the username to the ftp client (e.g. filezilla) you have to add the @domain.com .
For example
Host : mydomain.com / Username: ftpusername@mydomain.com / Password: mypassword
Edit: i also did this
http://wiki.centos-webpanel.com/pure-ftpd-setup-passive-ftp-ports
That's all for now