Remove rules and delete all mailboxes without deleting the email.

Hi   Bisal Ghale

Thanks for using Microsoft products and posting in the community. 

We understand that you have a question about the removal rule, please try the following steps to see if they resolve your issue.

• delete the custom mailboxes

If you are referring to a newly created folder, you need to manually batch move the emails in the folder to other folders and then delete the customized folder. Move or copy an item to another folder - Microsoft Support

1. To select adjacent messages, click the first message, and then hold down Shift and click the last message. All messages in the list between the first and last message you clicked are selected.

2. To select nonadjacent messages, click the first message, and then hold down Ctrl and click additional items.

If your "customized mailboxes" refer to inactive email accounts. If you want to delete the corresponding account, but need to keep the e-mail content on the local computer. You will need to export the emails first and then delete the relevant email account. Export emails, contacts, and calendar items to Outlook using a .pst file - Microsoft Support. Note: Exporting and importing a mailbox using a .pst file isn't supported in new Outlook at this time. To export a mailbox, you need to use classic Outlook.

• Deletion Rules

1. Click Rules.

2. Click Manage Rules & Alerts.

3. In the Rules and Alerts dialog box, click the rule that you want to delete, and then click Delete.

4. Click Apply

I hope this information has been helpful. If you have any questions, please let me know and I will be happy to help you further.

Forry-MSFT | Microsoft Community Support Specialist