How do I get my email client to check mail and quit as soon as it's finished downloading?

I’m running OSX on an older Macintosh G5 computer. I’ve found that leaving my Mail program open slows down my system.

After Mail.app finishes downloading, it plays a tone and then remains open. I want the program to quit as soon as it’s finished downloading mail so the system resources used by Mail.app can be freed up for other uses, even if I forget to manually shut down Mail.

Many thanks for any suggestions.