I think what OP means is that you can mix using an external email provider with storing your own email archive + an IMAP server + a webmail app. You can let the provider deal with the IP reputation and all that pain and just use their SMTP and IMAP to send email and pull to your local server, respectively. If you use your own domain you can also switch the provider in 10 minutes by simply changing your DNS records and retain the same address.

The hard part for me when giving up Gmail wasn’t the stuff above, it was tracking down all the places I was subscribed as @gmail.com and replacing it with @my.domain addresses. That took about 6 months. The local pull + IMAP + webmail took a weekend to set up.