Etiquette on sending a thank you e-mail to respondents who gave me helpful information Sometimes there are cases in a e-mail correspondence with academia staff (professors and bureaucrats) in which I don't know which is the best practice. I wonder if there is any best practice for the following cases:

Reply to a reply: when I ask for information via e-mail, and the reply gives me all the information I want, should I send an email just to thank them, or is this considered a bad, time-consuming practice? 

How does a professor react to this kind of reply? And what about a bureaucrat (who maybe receives more e-mail)?

If I asked for an internship and he gave me a negative response, is not replying to him considered rude or normal?
To send a short mail saying thanks for a service provided is never wrong; in fact, it is good etiquette. Sending such a mail also serves as a receipt acknowledging you received the information. I recommend a very short mail; do not overdo it, the show of gratitude is enough.