Take a look at this link and see if it helps
I've been using Windows Live Mail, but I'm with Comcast. I had to change my port to 587 though, and never had a problem. Try clicking APPLY before clicking OK and see if that helps. Occasionally WLM can get quirky and you have to uninstall and reinstall it although I haven't had to do that for at least a year. If you do that, but sure to save your contacts and any email you don't want to lose first.
Due to difficulties sending mail I talked to Att customer service and was instructed to change my outgoing mail port from 25 to 587. I also changed my ATT mail password. When I change the port or password in windows live mail, apply and say ok to the changes, close live mail and re-open live mail my changes have reverted back to the previous settings. Something seems to be blocking my changes. Att says it's not them and McAfee says their programs won't do this. I get a pop-up box with the old password when I first open live mail. Any suggestions?