I don't see any errors in /var/log/mail.log
I sent mail from my ISPs email to my newly created domain email and then logged into the webmail client and refreshed the inbox a few times. This is what the logs say.
Email addresses have been edited.
This is my home email address.
This is the mail login created by ispconfig
This is the mail address the original mail was sent to. It was setup through the options in ISPconfig also.
Here's the log, I can repaste it here if you don't allow linking to pastebin.