"To open the mail" means to physically or electronically open letters or packages that have been delivered in order to read or view the contents inside. Full definition
When you receive a new email notification and tap on it to open the message, it doesn't open the Mail app or message. (blogs.windows.com)
However, anyone that can open a mail box can settle debt, so why don't you try and impress us all with something that actually matters? (getoutofdebt.org)
We fixed an issue where receiving a new email notification and tapping on it to open the message wouldn't open the Mail app or message. (blogs.windows.com)