If you keep a lot of electronic letters on your disk, or if you have other disk files stored on the mainframe, you may eventually fill your mainframe disk to the point where managing mail becomes difficult. When you try to browse through your mail you will see a message telling you that your disk is full, and when you try to discard a letter, the letter is still there next time you read mail.
Mail can arrive at any time of the night or day over the network.
If you then quit from browsing mail, some or all of the new mail will remain in the holding area. NEW LOG A file. Clearly, the ultimate, long term solution to this dilemma is to keep enough free disk space to handle incoming mail. One solution is to ask UNET for some additional disk space. Normally, people have one cylinder of disk space.