Thanks Mario49.
Interestingly the location of the pick list has changed or at least it looks different.
The link in your message goes to a site apparently removed.
Perhaps it is being updated.
I would like to be able to click on the compose icon or new message menu item and have it come up with my most commonly used email without having to remember to go to the pick list.
Right now the default is @mac.com rather than my ISP. Having it as my ISP is my preference.
A couple years ago I had a similar problem and I was able to revise the list by deleting and re-adding the address I wanted to be the default.
I don't know if that would work now. The accounts have at least in part drifted to the system preferences pane for internet accounts and the order is the same in the email accounts preference tab for mail.
I can live with it, but find it annoying like the way calendar messes with appointment times with time zone travel.
It requires an extra step. Computers are supposed to ultimately make things simpler (or so I'd like to believe.)
Thanks again.