Unable to send emails to outdoor domains in Microsoft Outlook

We are currently using Lotus Domino Version 8 in our network, and our clients connect to us using Lotus Notes Client.
Recently, however, we facilitated the configuration of a POP3 access for our clients who wished to use Microsoft Outlook. Everything was working normally, except that an error occurs every time we send emails to outdoor domains.
Please see the attached image for the illustration of the error.
I have already checked all the confirmation settings in Outlook that might look similar to those that can be found in the Domino server so that the issue can be resolved, but I haven't had much luck.
Please help me find a way to solve this problem.
Thanks a lot, everyone!

OutIook Send/Receive Progress
0 of 1 Tasks have completed successfully
Dont show this dialog box during Send/Receive
Task 'PIC – Sending' reported error (0x800CCC80) : 'None of the authentication methods supported by this client are supported by your server.'
