Not sure what you mean by internal vs, external, but if it relates to workflow, please see below. If not, you could use search type or a an address book category to represent internal vs. external customer.
Issue 3: Credit Limit workflow messages not appearing in the Work Center.
Resolution 3: There are a few reasons this might happen:
The most common reason for this is that the Approver’s setup for Email/Internet in Address Book is incorrect.
Starting at 8.11SP1 release, messages can be sent to both external e-mail and the Work Center. It may be that only messages are being sent to external email. To correct this:
On Address Book Revision (P01012) inquire on the credit manager's address book record.
Highlight the record and select row exit 'Who's Who'.
Row exit to 'Email/Internet'.
Add a new line and use the drop down to select on Electronic Address Type = Internal Address.
Click OK to save
Note: If you don’t want both external and internal mail for Workflow messages, you can change the existing line, rather than adding a new one.