I can't imagine the cost would be significant, relative to the overall cost of running GMail. And it's not like they generally try to run GMail on the cheap. P

I can't imagine the cost would be significant, relative to the overall cost of running GMail. And it's not like they generally try to run GMail on the cheap. Perhaps they're concerned that seeing an 'undo' button would confuse people about how e-mail works, and they'd get a lot of requests to 'undo' messages that had really been sent. Or perhaps there are enough cases where people really want the message to go instantly that even 30 seconds delay would annoy them.