It seems that it is sometimes difficult to email an individual student using Outlook. If you've already used Outlook to correspond with a student then the email address seems to populate, otherwise it may be hidden in the directory.
I think I just figured out a way to do it to make it easier for you. I don’t have rostered students so these things are sometimes hard for me to troubleshoot, but this is what I did:
In Outlook, I went into "Groups" on the left side of the screen. I selected “ 12 members” (see below) and then chose an individual name from the list . I was then able to click on the email address and proceed.
Please note: All teachers should have their classes listed as Groups so if you would like to send an email to your entire class, just click on "Send email" (see below).
Please test this out and let me know if it works for you!!