How do you access the email client? Please be that it can not be accessed trough a website that you created in ispconfig as it runs on the default vhost. Yopu have to use e.g. the IP address of your server. As alternative, create a new website like webmail.yourdomain.com and install a webmail client inside this website.
Regarding the error in the mail log, it seems that your harddisk is full. Post the output of: