I think I missed something in your description, but what are you running on your local server? I think most people set up postfix to relay the emails over to gmail or whoever, and there are options in postfix for backwards compatibility with Outlook or even Microsoft Mail so your wife could use whatever client she wants. If you don’t have a local mail server set up then this is probably what you want to do. This method allow a local or remote connection from any client so you could run K9 on your phone instead of a VPN.
For opening such a setup to the internet (and allowing access from anywhere), make sure you have strong passwords on your accounts, require SASL authentication, and set up fail2ban to block repeated attempts to hack your mailboxes. Don’t run anything else on the same server (or use virtual machines or strong containers) to reduce the chance of your mail server getting compromised other ways, and you should be good to go.