The key factors that decide the time for an email to reach Mars include the distance between Earth and Mars, the speed of light, and the configuration of the spacecraft and communication systems involved.

The distance between Earth and Mars varies at their closest approach and at their farthest. It’s about 400 million kilometers at the farthest. So, to keep things simple I’m assuming the farthest case.

Email uses communication signals based on electromagnetic waves that travel at the speed of light and the speed of light in a vacuum is about 300000 Km/second

Now Time = Distance/ Speed, hence time = 400000000 Km/ 300000Km/sec

This calculation gives 1333.33 seconds or approximately 22.22 minutes, ie. the time it takes to send an email to mars.