I have a custom domain setup using outlook.com.  My wife and I both have accounts on it.  My account is working great, no problems.  My wife's account periodically can't send emails.  They show up in her sent items on outlook.com, but people won't receive them including emails sent to my email address that is on the same domain (and therefore on outlook.com).  

It doesn't seem to matter what email client she uses (web, wp8, w8, ipad, etc), they will all appear to work fine.  Emails will be in the sent items folder, but the recipient will never get it.  There's nothing in my wife's account to be able to differentiate a successfully sent email vs one that failed.  All of the emails that were set are visible in outlook.com in her sent items.