Possible Duplicate:
R: compute correlation by group

I have a data set with a 6 or so data points for groups of data.

e.g.:

email_id   date_sent  number_sent  number_of_views  number_of_responses
1           5/4         600            25                6
1           5/5         500            22                8
1           5/6         450            23                4
1           5/7         700            34               12
2           5/5         900            30               10
2           5/6         750            28               11
...

(this is made up data that illustrates the point)

Assuming I have this in a data frame in R, I'd like to write something which will give me stats by group. I'm most interested in the correlation coefficient between some of the columns.

I know how to do this with a data frame that contains only one group:

cor(col1, col2)

but I'd like to learn a technique that will allow me to extract data that looks something like this:

email_id    cor(col3, col4)
1             .73
2             .85
3             .98

and so on.

Thanks,