Email problems like ths can be tricky to trouble-shoot. Odds are it's an SMTP problem - as in, the mail server you're connecting to is flaky and doesn't want to send the email - but it might be a local client problem. Go into the Account settings in Outlook, and for that particular account there will be a "test settings" button. When you press this button it connects to the POP/IMAP and SMTP servers and tries to send an email. If it says it's successful, try it a few more times. If they all work, then that's a bit of a puzzle, because it means that your settings are all correct. Might be time to contact your ISP or whomever is running the mail server...
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