I don’t think the issue is public records. Certainly that is a problem, but the real issue is that anything he writes can be subpoenaed. A private letter certainly can be.
Given the attack nature of politics, he has to assume someone is going to court tomorrow to try to find some dirt.
I read an article on this that mentioned that Bush had been in the habit of regularly emailing his daughters – as president he gave it up because he did not want those communications to ever end up in the public domain. Sad, but as noted, this is the world we have made.
I could have sworn that Bill Clinton and Chelsea e-mailed each other while she was at Stanford; ISTR hearing about this on NBC news or something. The newscast was trying to present Clinton as having to go through the same things that other fathers go through when their daughters go to college.
Seriously, are father-daughter exchanges subject to FOIA?
If he used a government computer and/or an official government account, then it could be. Potentially, it could be if it went through government owned servers and routers. There are, however, plenty of delaying tactics which could be used (and have been used by many Presidents) to slow or prevent the release of such materials.