If return code is 28, check to see if there is data set contention for the data set name displayed in the error message. SMTP must have access to all its data sets to perform its function. Therefore, if a defragmentation utility (or any other application) is actively accessing SMTP data sets while the SMTP server is active, a data-set contention problem could occur. To avoid this problem, configure the application to bypass data sets with the SMTP high level qualifier. The high-level qualifier is set by the SMTP configuration parameter MAILFILEDSPREFIX.