If you are referring to the SonicWALL Secure Email Gateway Virtual Appliance this is how you get to the log files (I am guessing you want the SMTP transaction logs):
Log in as admin user
Click System | Advanced
About 3/4 down the page you will find Download System / Logs
All sorts of things are there. You will be looking for the MlfAsgSMTP.log or MlfAsgSMTP_#,log
The hash will be a number from 0 to 5. Each log grows to 50MB and rolls to the next log. The log 5 will be the oldest email in the system. The unnumbered log is the newest.
There are also a lot of system entries in addition to the SMTP transactions that occurred.