I was trying to set up "Send As" on gmail for my school account and even though I was certainly using the instructions on outlook, and the correct password and username I was getting "Missed upload deadline (9.9 sec)". After some searching I found a popular answer that I should be pinging "smtp.office365.com" and be using the result. That did not work either. As I found the answer I will be posting it below but please add a better one by all means.
The solution in my case was to VPN to a location close to where my school was and then ping. It seems that outlook smtp is located close to where your account was opened, not where you are at the moment.
As my school was in Melbourne (while I am in Athens) before the VPN I was getting
outlook.ms-acdc.office.com as the result of
but when I VPNd to Melbourne I got
outlook-au.office365.com which worked.
As I have lost a couple of hours doing that I am posting this here in the hope that it will help someone.
A app-specific password for your live account should fix the problem. Open: https://account.live.com/proofs/manage/additional?mkt=en-US&refd=account.microsoft.com&refp=security
Click on "Create a new app password" in the "App passwords"-Section, copy&paste the generated password to Gmail.