This question already has an answer here:
I'm posting this question because normally he is a very punctual person, often responding within a matter of hours if our time zones match up. However, I sent an email last week and a follow-up one on Sunday, and he has yet to read it (according to my email-tracking software).
I don't think his university has started break yet, so should I wait a little bit and leave a message?
Edit: Question should not be considered a duplicate because I am inquiring as to whether I should call him now that email seems to be "exhausted" so to speak, but I'm not sure if calling is appropriate.