Configuring preferences and options in Outlook.com
Yes you have 15GB INCLUDING attachments. That is specifically stated. But somehow there is someone within MicroSoft proposed to also add said attachments toward your onedrive. In any normal company, such an idea would be shot down and the person with that idea would be demoted. In MicroSoft however, this insane notion was accepted to be a swell idea. To say they lost their marbles would be an understatement.
I have had this emailadress for decades so I do not want to abandon it. So I deleted enormous amounts of mails with attachments and my Onedrive is now below 5GB and my mailbox is 7GB, so yes, you indeed DO seem to have 15GB of emailstorage including attachments, but the attachments alone are not allowed to be over 5GB.
The following calculation is very flawed, but I am making it anyway for entertainment purposes... My mailbox is 2GB over the 5GB OneDrive limit, so those 2GB are purely mailmessages without attachments. It took me over 25 years to collect those emails. So with the remaining 8GB, I am set to receive another 100 years worth of emails! Why am I even complaining about this GREAT company??? They just want to make sure even the condolances after my passing will still arrive at my emailaddress. Just make sure to send them without attachments...