I am having the same issue.  I was using DSL and everything worked properly.  I upgraded to U-verse and now I can not send email using Outlook through an external domain email account.  These accounts worked with DSL/Outlook - they are not working with U-verse/Outlook.  I changed the port settings to 587 as suggested but they still do not work.  U-verse says it is not their problem and want to charge me to fix what I feel they broke.   I'll take my business somewhere else before that happens.

Anyone have a solution other then dumping U-verse and going to cable?