Question about Outlook Express

There are 4 of us in the office and we all use Outlook for Mac 2011.  A few weeks ago, 2 of us noticed that suddenly the emails we sent through Outlook were not getting to the recipients (the other 2 are sending and receiving with no problem).  We get no error message, and it looks as if our emails have been sent (they appear in our sent box).  We found out they weren't actually being sent because several people told us they did not receive emails that we had sent them.  Incoming emails work fine.  Outgoing emails to the other 3 people in the office do work, but no one else.

Since then I've contacted our email service provider several times about this issue.  They've had me change the outgoing server info in my Outlook account settings.  I've tried changing those settings to read exactly like the settings of the 2 of us that are not having problems.  Nothing has worked.  It seems to be a Microsoft issue and I don't know what else to do. 

Any help would be greatly appreciated!