Good Morning,
we are testing the last version of Zextra ZxMobile on our Zimbra Server (version 8.5). Server is running on a latest version of Centos7.
we are having several problems with outgoing email. we are testing for now only 2 account but on our server we have about 100 users.
for tests we are using 3 different Android Phone (versions 4.2.1, 4.3 and 4.4). one of these is a Samsung Phone and the "Samsung Max EAS Version" is set at 2.5 (according to your specifications).
these is the problem: sometimes each phone send e-mail without problem, sometimes, randomly, the mail stay in outgoing messages on phone for long time. In this case: sometimes the mail is sent after many tries, sometimes the mail disappared from outgoin messages but it is not sent (we can't find it in Outbox and does not arrive at destination, even if recipient is on the same server!)
In the sync.log files we found a lot of java exception.
Can you try me? we are intrested to buy Zextra suite but we must be sure that everything works fine!
please tell me if you need some log file.
Sorry for my poor english! 
Best Regards,
Andrea Fabio d'Anzeo