Mac OS - This Connection is Not Private

Issue

Web browsers (Chrome and Safari) give warnings "This connection is not private" on certain HTTPS websites. After a closer look, it appeared that all affected websites were using Let's Encrypt certificates.

The computer in question was a 2011 MacBook Air running El Capitan Mac OS. The issue was caused by DST Root CA X3 root certificate expiration in September 2021. As explained by Let's Encrypt, this certificate has been replaced by ISRG Root X1 which is now trusted by pretty much all up to date operating systems. The problem is that El Capitan (and some other no longer supported older OS's) never received this root certificate therefore do not trust it.

Add HTML signature in Apple Mail

If you are lucky (and if HTML formatting is very simple), you may be able to copy and paste a HTML signature directly into the Apple Mail Signatures box. Just remember to uncheck the "Always match my default message font" box.

Unfortunately, if HTML formatting is slightly more complicated, Apple Mail is likely to mess it up. In this case, follow the below steps to insert HTML code directly into the signature file.

This copy of the Install macOS application is damaged

Issue

Trying to install a fresh copy of macOS High Sierra on a MacBook Air using a bootable USB key. The installation fails to proceed with the following error:

This copy of the Install macOS High Sierra application is damaged, and can't be used to install macOS.

Microsoft Exchange account stops working in Apple Mail - Enter Password

Issue

Microsoft Office 365 Exchange account stops working in Apple Mail on macOS. Apple Mail and Internet Accounts in System Preferences prompt to re-enter Microsoft Exchange account password, but after the correct password is entered, just shows spinning wheel icon indefinitely.