I have linked Thunderbird to my iCloud mail using Apple's straightforward app-specific password facility. Thunderbird recognises the basic folders (inbox etc...) and I can send / receive mails. Thunderbird does not recognise the majority of iCloud's Mail Folder hierarchy so most of my mail 'history' going back 20 years is missing. It is still present if I view it on iCloud's web service. Two random folders (each containing 30+ mails) are available through the Thunderbird client. The remaining 70-80 folders are not. This is a very simple Thunderbird set up, single e-mail account, no messaging or anything else (no need, don't want). HP laptop, Windows 11, Intel i5 - plenty of RAM and local storage