Use a subject line that would make YOU want to open it," is the advice Mike Evans of Relevant9.com. Mike provided me with the following example.
Which one of these would you open: “Get a FREE Website Audit!” or “Found a broken link”?
"If you were a regular person on their site and found a broken link, the last one is a subject line you might actually use. So when I write subjects, I try to think like the type of person that they would be interested to receive an email from."
Apply that rule to your domain emails. How you craft that subject is up to you and will vary from end user to end user and their particular needs. I can't write the magic words for you that will work in every situation. It's going to take some thought on your part based on the domain name and the research you have done on those you are targeting.