554 response is generally a rejection from the recipient server - this could be due to a filtering rule set up on the recipient's server, and is, again, generally due to the mail being marked as bulk or spam. There's not much you can do on your side to fix the issue, at least in terms of modifying or changing any SmarterMail settings. Many times these will clear up on their own.
One thing you may want to do is check to make sure your DNS is set up properly. Use a tool like mxtoolbox.com and run a test on your domain. There are a few posts in our Forums
that address this issue that you may find useful. Just do a search on "remote host said: 554" and read through the posts.