New user not receiving access email from WordPress

I am trying to give a new user access to my website in the “subscriber” category. He has not received the access email from WordPress, so could not enter username, password etc to gain access. (Equally, I did not receive an email from WordPress saying he had been sent an email.)

In the past I have successfully given several other people access (my website still being “Under Construction” and I wanted their opinions on various aspects of it). I used exactly the same procedure as now (“Add New User” etc), and it all worked then (except that I also did not receive the confirmatory email).

This current problem occurred a few days ago. Today I upgraded to 4.5.1, but the problem is still there. I am no techie but can manage the website itself perfectly with the aid of a splendidly simple book (not sure if I can mention the author’s name). So solutions please, but in simple language!