The next thing you can do is to search for and correct simple data entry mistakes like misspelled domains (geemail.com, hotmole.com, etc.) for example.
Having a few of these is quite normal, but if you are finding many bad domains then you may want to try and determine how those addresses got onto your list.
Do you have data capture issues? What can you do to eliminate the possibility of bad domains making it on to your database? Perhaps include reminder text on your signup form for subscribers to double check their entries, or add another email address field that gets recorded on a separate list, to cross-compare the entries when you are doing list hygiene checks.