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I am having this exact same problem and just like you it began very recently and only with outgoing mail on my pc.
the ipad email is not effected.
this is NOT a settings problem.
i have never changed them from the time they worked fine.
I am convinced that apple has changed the icloud SMTP server to not permit outgoing mail from a windows device.
Likely to force us to move exclusively to apple hardware.
Does anyone have any information on this?
I am also having this problem. I want to continue to use my @mac.com email address from a windows phone (Lumia 820) and a Surface RT. But have stopped allowing me to send.
APPLE please respond and confirm if you are blocking me from using a icloud account with non Apple hardware. If that is the case then I believe you will seriously upset a lot of customers who use mixed environments.