iMac software update

We have an iMac (21.5-inch, Late 2009) which is old but it is used for emails and other work. I personally am not very good with apple, i am a windows user, but my mother is subsequently worse with computers period. The built in mail app on this mac gives us a lot of trouble with sending and receiving, we are not sure why. We have tried downloading new mail apps but they require a software update, however when we try update to the software on the app store it says its not compatible. We would like to find out what software is compatible with this mac and the latest one at that. if anyone can help us it would be much appreciated.

Thank you