Responding quickly enhances the team feelings of good will
nEveryone will know Derek always responds promptly, so when he writes, “I am really swamped now but will respond as soon as I can,” the receiver will believe it and know he or she is not being put at the bottom of the pile; Derek will respond as quickly as possible and isn’t making excuses.
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nReceiving prompt responses to e‑mails tells the receiver that Derek regards that person highly.  Not responding or responding after days tells the person Derek doesn’t value him or her.  In other words, responding promptly builds good will. 
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