A number of people, both at ESPs and on the mailops mailing list, are reporting problems at Hotmail. The most common reports are senders getting
Site hotmail.com (65.55.92.152) said in
response to MAIL FROM (452 Out of memory)
during the SMTP session.
There isn’t much senders can do about this. Hotmail is going to have to fix their resource issue. This may be limited to certain mail clusters, so watching the IP addresses reporting this error and avoiding them may help.
Hotmail has recently stopped using Sender ID for email authentication and switched to authenticating with SPF. The protocol differences between SenderID and SPF were subtle and most senders who were getting a pass at Hotmail were already publishing SPF records.
From an email in my inbox from September:
There are a number of sites online that provide public information about reputation of an IP address or domain name.
Read MoreMicrosoft announced a number of changes to the Hotmail interface today. It doesn’t look like this will affect how mail is received, but will affect how users can interact with it.
As always, the best advice I can give you is send mail people want and like.