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Does this issue occur in Outlook desktop client?
If so, in order to further confirm the issue, please log in to web mail and send emails to check if the issue still exists.
If the issue doesn't exist on web mail, the issue may be related to Outlook desktop client. What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook to the latest version. And what's the type of your account?(POP/IMAP/Exchange)
As i know, this issue may be related to the send/receive error. Please open the Send/Receive Progress dialog to see if there is a backlog or any error report. It's suggested that you could change the Send/Receive interval to larger number(such as 5 min to 10 min) via File-> Options-> Advanced-> Send/Receive…
If the issue still exists, it is suggested that you could try to create a new profile(Control Panel>Mail>Show Profiles>add) and add the account for testing to check if there are some differences.
In addition, antivirus software may conflict with outlook. It is suggested that you could temporarily close your antivirus software and restart outlook to check whether this problem still exists.
Hope the above help!
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