You created an alias of the Mail app and then moved the alias to the dock, you can drag the alias out until it becomes a wad of paper then release the mouse to remove it from the dock, then go back into your applications and drag the Mail app down to your dock in it's place.
[doublepost=1460765496][/doublepost]I did as you suggested when this problem first appeared but on dragging the Mail app to the dock it once again appeared as an alias (the arrow). I was careful not to pause on the desk top, to make sure that I wasn't creating an alias, before letting it go on the dock. This problem is on the Macbook of a friend which is why I am a bit slow in replying to these posts.
Now here is another peculiarity. When I visited this friend a couple of days ago the Mail icon (still with the arrow) had acquired a question mark superimposed upon it and did not open the Mail app. I dragged this icon off the dock and dragged another from the applications folder on to the dock which again displayed the alias arrow but did, at least, open the Mail app.
All very odd, isn't it?
The Dock icon is probably pointing to an alias. Ctrl-click on the Mail icon in the dock (that's an alias) and select "Options->Show in Finder" to see where it is really pointing. Then select the revealed item, use "Get Info" and see where the original file is.
[doublepost=1460766209][/doublepost]Thank you for this suggestion. i will follow it up at the first opportunity and let you know. As you may gather, from my reply to mildojr, this may take a day or two.