It sounds like your Outgoing Mail Server (SMTP) is not set up correctly. I would suggest ypu open Mail>Preferences>Accounts>go down to the line on the lower right of the window that says "Outgoing Mail Server (SMTP)>click on the drop down arrows at the right end of that line, and click on the "Edit SMTP Server List..."> in the following two available windows make sure all the settings are correct as to Port, Password, Server Name, etc. as stated by your ISP (in this case, AOL).
I've managed to sort it, it was the weirdest of things.
In the below screen shot where the arrow is indicating. I had my christian name and surname in this section. I then changed it to my full AOL email address and its sending and receiving emails fine now
Thanks very much for the help, its much appreciated.