You can quickly filter emails by a flag with the Instant Search feature in Outlook. Please do as follows:

Open the specified mail folder you will search emails within, put the cursor in the Instant Search box to activate the Search Tools, and then click Search > Flagged. See screenshot:

Now the search criteria followupflag:followup flag is added in the Instant Search box, and all flagged messages are found out and listed on the mailing list. See screenshot:

Quickly search and delete duplicate emails in Outlook

With Kutools for Outlook’s Duplicate Emails feature, you can quickly find and delete them from multiple mail folders, or find and delete all duplicates from the selected ones with two clicks in Outlook.

You can also create a search folder to collect all flagged messages in Outlook. Please do as follows:

1. Click Folder > New Search Folder. See screenshot:

2. In the New Search Folder, please click the Mail flagged for follow up option, and click the OK button. See screenshot:

Now a new search folder named as For Follow Up is created with all flagged messages listed inside. See screenshot:

Add follow up folder to favorites in Outlook

Remove follow up flags for email messages in Outlook