Hi,
Messages has been able to add Jabber accounts since it was the previous App iChat and version 3 of that.
In fact the versions numbers for Messages continues on from iChat with the Lion version being ichat 6 and Mountain Lion having Messages 7
This is the list of Jabber servers http://www.jabberes.org/servers/
It is not exhaustive by any means but it does give you an idea.
Google is not even mentioned in that list but they do run a Jabber server.
This is my settings for Google.
Despite me editing the com.apple.iChat.Jabber.plist to allow it to be reset to "talk.google.com" and SSL ticked and port 5223 which is the one for SSL logins it currently always overrides anything I put in and show this.
Each time I start up messages I get this Alert
This ties up with the second grey tick in the picture above about warning about sending info Insecurely.
The issues I have with Google have only started in the last few weeks.
For a while now they have tried to shift people away from SSL (Secure Sockets Layer) logins to their Servers.
They use their own Oauth Tokens instead.
So you have to set your Google Account setting to Allow "Less Secure Apps" so you could use SSL.
In the meantime the Apple Mail app has continued to use SSL with Google's servers complaining at all.
Different Jabber servers login with and without SSL generally down to the Server choice (If they do one they don't do the other.)
As I said before knowing what type of Jabber Account you have is likely to be the first step to solving this.
You still have not said either way.
You will notice that I have edited some of the personal info in the Pictures I have posted.
If you post some of yours you will need to do the same but is may make things quicker.
7:46 pm Monday; July 11, 2016
iMac 2.5Ghz i5 2011 (El Capitan)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone and an iPad (2)