After getting my Intel NUC at the end of November and setting it up with Windows 8.1 and Office 2013, I left it as is until Santa could deliver me a new external hard drive for storage purposes.

After setting up everything else, I tackled Outlook 2013 and setting up my Outlook.com powered custom domain.
It took a bit of searching to find the server name (m.hotmail.com), but everything went smoothly and emails started to sync.

As I had to head back to work today and needed something to pass a few minutes, I decided to send myself an email with a small Word Attachment (20kb). New email -> to me -> add attachment and send. It sat in the outbox for over 7 hours before I gave up.
I tried again and same result. I then did a new email -> to me without an attachment. It sent straight away.

I logged onto Outlook.com and sent myself the same email attaching the file from my system and it had no problems, arriving in my Outlook 2013 mailbox quickly (as well as on my phone and Outlook.com)

Has anyone seen this before?
I haven't had time yet to test attachments to other email address aside from my own, but will over the weekend 

At the moment I am thinking that a downgrade back to Office 2010 is in order, as this connection worked perfectly