How to troubleshoot cdo email

My CDO mail routine has been working great for several years. The user fills in from and to addresses, subject and body, then before hitting Send button, must choose their own credentials from a table (where they have previously entered username, pw, outgoing smtp, port, etc.). This circumvents some isp's sending restrictions. I have just run into a user whose smtp is smtp-server.nc.rr.com. Last week even her Outlook suddenly failed to send, so she contacted the help desk who had her change from port 25 to 587 and check Auth and Outlook started functioning normally. She then made those changes to the table of sending credentials in the database program, but it fails. She has experimented with all combinations of ports and auth checks but nothing works. I am in need of where/how to begin to troubleshoot this, particularly since I don't have access to her isp. Help, please?

Try my CDO email tester.
https://www.access-programmers.co.uk/forums/showthread.php?t=293368

It will allow you to test various settings and if there is an issue, the help file should help you understand what the issue is