Outlook sending emails but recipients don't receive them and no error message

Now for a couple of months I'm having the problem with sending emails from Outlook (Office 365).  The email shows that it has been send and show in the SEND folder but recipient never receives it and no error message that it was blocked by a mail server.  Sometimes if I send the email 2 or 3 times in a row one will reach the recipient. I have a couple of my clients complaining about the exact same problem.  I have tried numerous things, including notifying my ISP and according to them the email was send.  I looked on forums and I've found that it's a common problem, but no solution that works.  I've even had the recipients add my email and domain to their whitelists and it goes good for a couple of emails and then we are back at square one.  I don't send that many emails that my domain and email could've been flagged as a spam sender.  Even checked that option and nothing.  So if someone can assist I would be grateful.