Outlook Clients are connecting to the wrong Exchange Server

We have a large complex domain with several sites.  Each site is connected via VPN and also has its own domain, DNS and mail server to support the users at the site.  In addition, with the exception of one site with two facilities, each site has its own mail domain name but from an AD perspective, they are sites within the same forest.

We recently started to have an issue were in one site, our outlook clients were connected to the email server in another site.  We can manually change the proxy settings and outlook will function but when you close Outlook and reopen it, it goes to the other  site.  Now we are having the issue at another site as well.  When the problem occurs, all of the clients get set to the mail server at the top of the primary site.  We have a combination of Exchange 2010 and 2013 along with Server 2008R2 and 2012.

DNS looks to be correct - but we just can grasp what his happening.  Some details:

Site 1 - mail.site1name.com (internal resolution to site1server.domain.local
Site 2 - mail.site1name.com (internal resolution to site2server.domain.local - second facility under this site)
Site 3 - mail.site3name.com (internal resolution to site3sever.domain.local
site 4 - mail.site4name.com (internal resolution to site4server.domain.local)

each site has an autodiscover.siteXname.com entry in the DNS for that site as well.  This entry does resolve to the internal mail server at that site - so, autodiscover.site4name.com -> site4server.domain.local

Problem:

user at site 3 configures outlook for msstd:site3server.site3name.com.  All is good until the user closes outlook.  As soon as they open outlook again, the configuration switches to mail.site1name.com.  For users to function, they have to manually correct this and then they can access their email.  This was working just fine, but some event changed how this was functioning for one site and now it is happening at another.  We can't put our finger on what could have caused this, but we do need to fix it.

Any assistance would be greatly appreciated.