There are lot's of reasons which are resposible to message sending failed in Outlook account. we are sharing a list of all that reasons with solution so you can read it and get overcome this problem as soon as.
1) Incorrect Password- it sounds simple when users are not able to send the messages due to password. Mostly people complain that they are not able to send the messages or mails, ensure that the password is correct or not.
2) Server Error- not happens all the time but most probably due to network connection problem user didn’t send their mail. To avoid such situation, kindly restart the service from the control panel and hopefully it works again as before.
3) Internet issue- sometime due to internet connection issue this happens. Make sure if you lose your internet connections then you are no longer to send the messages. To get rid of this problem, try this site http://www.microsoft.com if you not able to reach on this website then troubleshoot your internet connectivity problem.
4) Larger Attachment- suppose you want to send any mail to your friend, colleagues etc with large attachment but at the same time outlook prevents you to send that attachment because of large attachment. If the attachment size is larger than 5Mb then surely this problem has appear. To avoid from this situation, delete that attachment or attach a small file.