You might like to see this thread...others are having similar problems.
https://www.mobileread.com/forums/sho...d.php?t=105784
The latest Calibre update (7.29) includes a very helpful Hotmail shortcut alongside the Gmail one ...under "Preferences, Sharing Books by Email".
Haven't used this yet. I continued to have difficulties this morning, and reverted to my Gmail address without success. It may have been due to the size of this morning's Guardian paper (7.4MB) as the Independent and NY Review of Books seemed fine.