Suddenly, last week and this week, I am getting the error "delivery status notification (failure) on outdoing emails that I send to valid email addresses.  No matter how many times I try, I get the failure notice.  I have emailed to these addresses before.  Please note, this is not happening with all emails, just a select few so some are getting through and some are not.

I am a Hotmail conversion to Outlook.com.  I have a primary account and a secondary account which I have "linked" so that my secondary emails now come into my primary account.  I also have the option of of using my primary or secondary email address in the "from" title.  I have noticed that these failures are happening when I try to use my secondary email address in the "from" area to reply to an email or to create an email from my primary account.  As a test, after I could not reply to an email in my primary account using my secondary "from" address, I logged into my secondary account and replied.  Surprisingly, the email went through with no failure.  So, I am wondering if the failure notices I am receiving are somehow caused by my "linked" accounts.  Any and all help is appreciated.