G'Day Dot
Dont worry about it, it is just one of the disconcerting gotchas for those who have not been caught by it before.
Just for future reference, these sort of headings (where you can click on them) are known as "clickable headings" so you can sort mail, folders, files, whatever into whatever you want, date, filename, subject etc. The first time you click on a heading it will sort into order based on that, the second time it will reverse the order, the third time will reverse the order again etc.
What probably happened is that you accidentally clicked on the "From" heading so your mail was sorted by sender, if you had clicked on "From" a second time the order would have been reversed.
All you did was tell it to resort your mail by "Date" instead of "From"
You can do exactly the same thing for files/folders in Explorer; eg open a folder and click on Size - this will sort the files and folders into order by size (smallest to largest), clicking on size a second time will reverse the order. If you then click on Name it will resort the files/folders into name order.
Geoff