One point may be to don’t use smarthost and fetchmail (or getmail) anymore and use direct mailing instead to be independent from the ISP but I don’t know if your ISP supports to enter DNS entries. For your case with one account it’s fully ok to keep everything as it was. Here are just some thoughts about smarthosts and what you could do on your own mail server.
If you don’t need to share files I’d recommend LDAP: