The thing is you often quote a mail like this:

Hi Pa!

You wrote:
A long sentence which
> went like this.

This is not correct it should be:

Hi Pa!

You wrote:
> A long sentence which 
> went like this.

All you need to look at the mail before you send it and check that the '>' signs are placed against all sentences which the other person sent you.

You can read up on e-mail netiquette by looking it up on Google!

Now I can see that are asking yourself whether it is important. Well, one usually writes e-mails to communicate. In communication there is give and take, as the other person should not have to make too much of an effort to figure out what is being said (and not too little either)! That is why there are conventions about spelling, punctuation, grammar etc. Not because some teacher/grammarian thought up some horrible rules to make life difficult for everyone but in fact to make life easier for everyone!