Letter writer snapped at my reminder email. What can I do in the future to prevent this, and should I still accept his letter? I sent a reminder email one day before the due date for a couple of my applications to grad school. It said something along the lines of 


  This is just a reminder that first due dates are tomorrow: [list of a
  couple of schools]. Hope you aren't too busy to write still. If you
  haven't received an email from these schools on instructions let me
  know.


I also sent "resends" to this professor's email via the schools' application portals. The professor responded with something like


  You've already sent a list of deadlines [I did, but this was about a
  month ago]. It is NOT helpful to send repeated reminders.


I thought it was standard to send reminders before deadlines, especially if due dates are tomorrow with no letter sent. I feel that I have said or done something wrong. How can I prevent this from happening in the future?

I am now not only reluctant to send reminders for January schools, but I am now worried about the strength of this professor's letter. I have four letter writers, and I am tempted to drop his letter on some January schools. The reason for this is that this is not the only time he has been a bit snappy with me, although never like this. But I did tell him a while ago that I did not want his letter if it would not be strong, and yet he still agreed to write for me. So assuming he is honest about that, I feel I should maybe keep his letter. 

This is all a little confusing and stressful. Thanks for your time to read.
How can I prevent this from happening in the future?


Well, you can always go live on a desert island, or join an order of silent monks (and even living among monks would not guarantee that you will escape such foibles of human nature).

More seriously: you can’t prevent it, and you shouldn’t try either. You did absolutely nothing wrong by sending a reminder email. For every professor who responds irritably to your emails there are at least ten others who would appreciate your reminder email, and for some of them the reminder would even be needed to get them to upload their letter. Thus, you stand to lose much more than you will gain by changing your behavior because of the professor’s response. Just accept that working with other people necessitates occasionally having some strange and perhaps unpleasant interactions (trust me, you will have many more), and move on. 

By the way, I also think it’s highly unlikely that the professor will modify his recommendation because of your email — that would be an extremely unprofessional and unethical thing to do, much more so than replying irritably to an email. Anyway, good luck with your applications.