I have this file on my Ubuntu server, and it is 12GB in file size. I don't know what that file is, and would like to know, would be safe to to delete or truncate the file?
Here is the file
Looks like that's a mail logfile for Zimbra/postfix (mail server).
It depends on the IT policy for the server. If it's not a business server, then do whatever you want if you don't need to troubleshoot any mail logs. If it's a business server, depending on the type of business, you may need to archive and keep the data for 7 to 10 years, unless your IT policy states you only keep logs for X amount of time.
Otherwise, you have free reign over it.