I managed to solve this one. Just for the sake of others to come I am going to put some lines here on how I solved the problem.

The "Unable to create a dot-lock at ..." problem was because I did not have the maild dir for the postmaster user. I therefore created this directory (in my case under /var/spool/mail/virtual) using the command mkdir.

This is bad because it gave me an error "Unable to open mailbox". After doing some search on the internet I found about the command 'maildirmake'. I deleted the previous postfix directory and created a new one using this command (you know it works because under the durectory you created you'll find other directories, such as 'new', 'cur', etc).

After doing this everything worked fine :) I know some information above may seem basic for expert users, but I think for a newb like me it is not :)