If you need to find out all external emails which are not from internal domain within POP email account in Outlook, please do as follows:

1. Shift to the Mail view, and open the specified mail folder where you want to search external emails.

2. Place the cursor into the Instant Search box, and then click Search > From.

And now the search criteria from:”Sender Name” has been inserted into the Instant Search box. See screenshot:

3. In the Instant Search box, please replace Sender Name with your internal domain, such as @extendoffice.com, and then add NOT before your internal domain.

Now the search criteria  from:”Sender Name” is changed to from: NOT”@extendoffice.com”. See screenshot:

So far all external emails have been found and listed in the mail list already.

One click to find emails by selected email's sender or sender domain in Outlook

With Kutools for Outlook, you can quickly search all email messages by selected email's sender with the (Search by) Sender feature, or search all email messages by selected email's sender domain with the (Search by) Sender Domain feature.

If you need to find out all external emails within Exchange account in Outlook, there seems no good ways to solve it. However, I will introduce a trick to distinguish external emails and internal emails in Exchange account’s mail folders.

1. Shift to the Mail view, open the specified mail folder of Exchange account you will distinguish external emails and internal ones, and then click View > View Settings.

2. In the opening Advanced View Settings dialog box, please click the Conditional Formatting button.

3. Now you get into the Conditional Formatting dialog box, please click the Add button, next type a name as you need into the Name box, and then click the Condition button. See screenshot:

4. In the new opening Filter dialog box, please type /O= into the From box on the Message tab, and then click the OK button.

5. And now you return to the Conditional Formatting dialog box, please click the Font button to open the Font dialog box, next customize the font formatting as you need in the Font dialog box, and click the OK button. See screenshot:

6. Click OK > OK to save settings and close dialog boxes.

And now in the specified mail folder, all emails with the customizing formatting are internal, while emails in normal font are external. See screenshot:

Search with wildcard in Outlook

Search in email messages body/text in Outlook

Search for an exact match to character strings/phrase in Outlook