There are many reasons why this can happen, and the key is to always check the actual error you receive when this happens.
#1) It could be that you may have entered the wrong email by mistake. Most likely not, but it does happen, so always double check the email you sent it to. It will usually say this email does NOT exist or is not in an alias table (which basically means it does not exist)
#2) If the error you get says, 'Can not write to users Output File' or 'This User is over quota', then that means that the person's email storage is completely full. This can happen with our server, google servers, yahoo servers, etc. The user needs to delete or empty their mailbox in order to get more emails again
#3) If you get couldn't be delivered due to suspected of being spam (see below example I created)
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server.camelotcomputers.com rejected your message to the following email addresses:
Robert Hightower (rhightower@ncaarbh.org)
Your message couldn't be delivered because it's suspected of being spam. For best practices when sending email, refer to the guidelines found here: https://go.microsoft.com/fwlink/?LinkID=526654.
server.camelotcomputers.com gave this error:
Decision Engine classified the mail item was rejected because of IP Block (from outbound normal IP pools) -> 554 5.7.1 Service unavailable; Client host [40.107.94.138] blocked using bl.spamcop.net; Blocked - see https://www.spamcop.net/bl.shtml?40.107.94.138
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This reason occurs because Microsoft Office 365 Servers were 'flagged' RED by the World Wide Internet spam watchers. In our example above, it was SpamCop.net that sent out a warning about Office 365. Basically, Office 365 sends out a lot of regular (good) emails but people's accounts all over the world can get hacked or infected or have problems and start acting like spam. Office 365 works around the clock to stop and delete this however, quite often, they break that 'red flag' threshold and get temporarily RED flagged. A typical red flag is 17 hours long. During that time, Office 365 SENT emails will have much more scrutiny than normal and emails have a much higher probability of being blocked, and thus, you receive that message you see above. After that time expires, the emails are fine again as Microsoft takes care of the problem as quick as possible.