I do a lot of IT work at my employer and had some strange things happen today with a couple people's Microsoft Outlook. Both their mail programs stopped working and was asking them for new login information, like their password was wrong. They had both been using Outlook for hours when this occurred. Both assured me they had not changed any settings in mail and wouldn't even know how to do it. After diving deep into the settings I found that their incoming and outgoing port settings had changed. It really confuses me as to how this happened since our mail server settings haven't changed in many years.
