Hi,
Same thing happening to me as well. Been sending for over 2 years across same IPs without any problems and all of the sudden they are rate limiting and blocking. They responded with the 421 error and also stating that some blocks are temperary and will be lifted soon, but seems still rate limited majorly.
Here is what they responded back with:
Windows Live Hotmail limits the number of email messages a particular IP can send within a time period. Based on an IP's reputation (built based on various data sources) it is allotted an allowed sending limit per unit time. When an IP (sender) exceeds its allowed limit, any further SMTP commands from the IP will receive the SMTP error code 421 from Windows Live Hotmail and the connection terminated.
The block against your IP is set to be lifted. At that time, you will be returned to the appropriate throttling level based on your reputation. Please note that this does not guarantee that your mail will be delivered to a user's inbox, only that it will no longer be throttled until it again exceeds its allowed limit.
BTW, whats SRD?
lc