The best response to the question about whether testing the email client with external email accounts will provide valuable insights into the product’s ability to meet the organization’s demands is:
Yes, testing the new software with external email accounts would be a plus point.
This response recognizes that testing the email client with external accounts can provide insights into how well the software interacts with emails that may come from outside the organization, which is important for any email client, even if the primary focus is on internal communication. External testing can help identify potential issues that might not arise in a purely internal testing environment, such as email formatting, security concerns, or deliverability problems with different email providers.