Well, the first thing you need to do is identify how you will determine whether or not your table has a new row (inserted).

Do you have any date columns on this table? Is one of them a created date for the row? Can you verify that the information in that column is only modified when a new row has been created?

If so, then you could do something like:

If Exists (Select * From MyTable Where datecolumn >= dateadd(minute, -30, getdate()))

Begin;

-- code to send mail here

End;

If you don't have any date columns to identify this - then you might be able to do something with an identity column if you have one on your table. This would involve something like:

Declare @seed int;

Set @seed = (Select seed From LastSeedTable);

If Exists (Select * From MyTable Where identityColumn > @seed)

Begin;

Update LastSeedTable

Set seed = (Select max(identityColumn) From MyTable);

-- code to send email

End;

You can then put the above in a stored procedure and call it from a SQL Agent job - or, just put the code in the Agent job.