The errors that you posted are ok, they do not mean that anything is wrong with your setup. They should accour as it means that someone ahs tried to send emails to Linux system users which dont have a mailbox.
So back to your original problem. Login to webmail and send a email to the same address that you used to login to the server and then post the lines that got added to the mail log file. Additionally, I need to know which tutorial was used to install the server.