I am aware of the basics. I have been using Mail since it was released to an unbelieving world as .Mac over 19 years ago and iCloud since it was first established. I did check the obvious. The first thing was to check my other devices and then that my wife’s Mail was working on her iPad running iOS 14.4.1, which it was. Safari was also running fine, which indicated there was no problem with the network or with the ISP. I also checked iCloud status regularly at the same link you provided and all services were operating normally. Curiously, my Mail was operating on iCloud.com but not through the Mail client.
That suggested that the fault was with the IMAP/SMTP server routing and it is not inconceivable that there are different servers dealing with legacy .Mac accounts to those dealing with more recent versions. I therefore offer to you the hypothesis that this is indeed the case and that Apple’s system status only indicates current and not legacy services. Would anyone like to share their thoughts on this?