hi tim I have logged into my 1&1 account and found the following message, it may help with any other queries that may be posted, dont know if you can post it as a sticky
"Intermittent email issues to Hotmail, outlook.com and live.com addresses.
Email sent to hotmail, outlook.com and live.com addresses may be intermittently rejected due to Microsoft blocking some of our IP addresses.We are aware of the issue and have contacted Microsoft to request de-listing. As not all emails are rejected, we kindly ask you to resend your email again. Please accept our apologises for any inconvenience caused."