George is exactly right.  PTO can e-file returns for taxpayers with foreign addresses.  Our clients are all expats and we've never had any problem e-filing just because the client has a foreign address.

If you subscribe to Docusign or other similar services for F.8879 digital signatures, you should consider an alternative as KBA may not work for your overseas clients.  The simplest solution is probably be to fall back on electronic signatures.  Some client portals do offer digital signatures but you will need to manage KBA differently.