Email address is series of long digita @outlook.com and not my actual email

I have been using outlook.com for my emails for some time and have a work account connected to it which is an SMTP account. Since last week my emails now send from an email address which is a long series of email address removed for privacy reasons and it says sent on behalf of hannahchatfield (my name). Some people are not receiving emails and others have the email bounce when they reply to it. I have attempted to add a new alias, remove the one with the error and add it back but the same thing happens. I cannot see how to fix this - can someone please advise?