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Please kindly understand that the Outlook tag here we mainly focus on general issues about Outlook desktop client. Given that you are using Optimum email, you may need to contact your email provider if you need to manually configure your account. The following preliminary troubleshooting is for reference only.
In order to better study this issue, please provide more details:
Can you provide a screenshot of the pop-up window with the error message? Please be careful to override personal privacy.
What type of account is added to Outlook (POP/IMAP/Exchange) ?
What is Outlook version? Please make sure to update to the latest version.
Based on my search, this issue may be caused if your server settings are incorrect or if the configuration file is corrupted. Here are similar threads for your reference:
Hope this helps!
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