That's even assuming the email really did come from that account - many of these are "spoofed" so that they *appear* to come from that account when they actually come from elsewhere.Or they could also be email addresses hijacked from real people. If you are lax about your password and firewall security then miscreants can sometimes get into your machine and copy your address book - they then spoof emails to appear to be from those addresses.
But don't get hung up on the Outlook Email thing. If you have a windows 7, 8 or 10 PC you have an Outlook email account - it's bound to your log-on user name. If you have an Android phone or tablet then you have at least one Gmail account - it's bound to the phone number. Many people don't use them and some don't even realise they have them, but they exist. The email providers do their best to block the spammers by blocking their IP addresses, but these can be changed/masked/spoofed as well. So they can no more guarantee to block spam email than the post office can block junk mail. It's part of life - sorry!
PDR