OJS 3.1 send review request problem

I am using OJS 3.1 and now holding a conference and with many articles ready for review (more than 1000).
Here is the problem I encountered.
Some reviewers were assigned from the system, but they never received the emails. I checked the email supplier and found also email failure.
Particularly, one reviewer has been assigned to three articles but he only received 2 emails.
I do not know what is the problem, can anyone help me here?
Thank you very much

Are the three requests from the same journal? If not, is it possible that the reviewer has more than one account (with different email address)?

@ajnyga they are from the same journal. all papers are in one journal.

and you checked that if you search for example with the reviewers last name, there is only one account and one email for the user?

@ajnyga yes, I am thinking if the reviewer’s email server or firewall blocked the email…but I dont know why it only blocked 1, and left the other 2.

yes exactly, it does not make sense. That’s why asked if the user has two accounts and therefore two email accounts.

It sounds more like something that is happening in the users email server and not so much in OJS. There is always the possibility that the user has removed the message accidentally. Or just some malfunction in the mail server I guess. Hard to say.

@ajnyga you are right, I just simply put a question to see if any has the same issue. I dont know if when this system handling more than 1000 papers in one journal at the same time, would some of the auto emails not sending???