Today I suddenly found that my email server requests a read receipt every time when I send emails. Since I am recently exchanging messages with a potential Phd supervisor, I wonder if this will make him feel that I am rude and annoying? I have already turned this feature off!
Unless you have reason to suspect that emails are not being delivered successfully, email receipts are pretty much useless. Here's why:
Not all email clients support read receipts - Mail on OS X doesn't, for one prominent example. If some of the previous emails you sent requested a receipt, and it wasn't returned, obviously either the prof's email client doesn't support it or he didn't bother clicking that button.
Unless the receipt is returned, you can never be sure if the email was read or not.
Unless the email is replied to immediately - which is unlikely if the reply will take some effort - it can easily be forgotten about. Receiving an email receipt is no guarantee that you'll get a reply.
If you have no response to an email after a reasonable period of time ("reasonable" can vary between 3 days and a month or more, depending on what is required), just send a quick reminder email.
I do not think that this is rude. If it even mildly irritated him he might have mentioned it to you at some point. Even if he didn't, he still corresponds with you so it probably didn't bother him that much. Regardless, if you disabled the feature, he should not be getting those any more.