From postfix's logfile I can grep all the lines that indicate the delivery of one message:
Jan 3 15:28:21 mail postfix/pipe[1040]: [...] status=sent
Jan 3 15:28:21 mail postfix/pipe[1157]: [...] status=sent
Jan 3 15:28:22 mail postfix/pipe[980]: [...] status=sent
I'd like to group them together by timestamp, so I can calculate the messages per second for a given time range. How can I do that?