My first couple of days of submitting to agents were exhausting and extremely frustrating. I would identify a target agent then send her (90% are women) whatever she requested. About a quarter of them came back immediately as undeliverable. Thinking, I guess, that "this agent was my only hope" I'd keep trying to find a better email address or resending it. Finally, I realized she may be out of business or posted a bad address and moved on.