SMP Press is a self-publishing program, which means you both retain the rights to your arrangements and that you are responsible for ensuring that you have all rights required to sell your music. Sheet Music Plus sells music all over the world. So, from a practical perspective, if you are asserting you have rights to arrange music because the composition is in the public domain, it will need to be in the public domain worldwide.
The general copyright durations for major markets areas are below.
United States: the work must have been composed before 1925 to be in the public domain.
United Kingdom, European Union, Australia, and a number of other countries: copyright duration lasts for life of longest living composer/arranger/editor/librettist plus 70 years.
You will need to evaluate your piece by the longest copyright duration to determine if it is in the public domain.
Sheet Music Plus is no longer maintaining a list of musical compositions in the public domain. www.pdinfo.com is a great source for public domain information in the United States and for researching public domain materials. Before you do any arranging, we recommend researching the musical work in question to confirm if it is in the public domain. If you are unable to confirm its public domain status, or if you have doubts, you should seek permission before using the work or consult a copyright attorney.