On a mailing list I subscribe to, someone recently asked for assistance with getting mail delivered to Microsoft's Live.com and Hotmail.com domains. Apparently the poster works for an internet provider that had a compromised account or two, and Microsoft was blocking their mail as a result.
A kind soul posted this reminder in response:
"The most efficient and effective way to address any deliverability problem is by submitting the issue to our dedicated deliverability support team. Senders can do this by filling out a form with detailed information necessary to diagnose the problem. The form can be found here: https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_22.214.171.124&wfname=capsub&productkey=edfsmsbl3&ccsid=. Going directly to deliverability support will ensure that we have the right information to investigate the cause and recommend the right solutions quickly."
Those of us who have been around a while already have that URL bookmarked, but I thought I would share it here for new folks who might not already know it.
(June 2015 update: Link has been updated to most current version.)