Hello Debalina Das98,
Good day!
Thank you for posting to Microsoft Community, we are glad to assist.
According to your description of Classic Outlook for Windows / For business,
remove the shared mailbox from Outlook.
Regarding your issue, if it was added manually, you should follow these steps to remove the shared mailbox.
Open Outlook > go to File > Account Settings > select your account > go to More settings > select Advanced tab > under Mailboxes, check if the shared email account is visible there, if yes, select the Remove option to remove it.
If a shared mailbox account is not visible in the Advance tab, it should be added in your Outlook with an automapping feature.
When we have full access permission on a shared email account and if automapping is enabled, then it will automatically add to the user’s Outlook.
In such a situation, to remove a shared email account, you need to disable permission first and re-assign permission with disable automapping. For this, you need to contact your Microsoft 365 admin. For details, please check Remove Automapping for a shared mailbox - Outlook | Microsoft Docs.
Best Regards,
Community Moderator