Outlook will not send/receive email on Mac

Thank you for responding and trying to help.

I've tried testing the mailbox in webmail, but my provider's webmail link is not working.  That's an issue I need to address with them, of course.  I've called them earlier, but will be calling back today.  I've also checked for messages stuck in Outbox and cleared all of those.  

I have never understood Keychain so have avoided using it, but this morning a dialog box randomly popped up asking for my login password, so I tried a couple and apparently got the right one because suddenly I got around 250 emails, including the email that linking to your response, which is particularly frustrating because obviously I would never have seen your suggestions if the random pop-up hadn't happened.  

Now it seems that I can receive but not send emails.  There is also a long gap between the messages I got today (from July 2 through today) and the last one I had received around the end of June.  I'm pretty sure thre are many messages sitting out there in the ether somewhere.

I have no idea how to remove the account from Outlook for Mac, and I'm not even sure what "Microsoft/Exchange credentials" are.  That's how un-techie I am.  I'll keep trying, and would appreciate any suggestions.