Why? Because you clicked on SENDER and had it sort on that column.

Click on the header labeled DATE. Click DATE a second time to reverse the sort order. This works on any column header.

junie506 said

until today my emails were displayed by date received with most current ones at the bottom of the list. This has changed to display in alphabetical order according to the sender. Why????? I need it to change back to the original.

thanks for the info - I solved it on my own !!!!