Hmm, i didn't try to install last version of postgrey plugin on an old i-MSCP. That makes no sense. I use the old plugin version 1.1.1 wich runs in a fresh install of 1.3 without any problem.
I just follow the few hints to migrate from one server to another, wich i found in the forum, ok, not the latest stable version, just the "old stable" version 1.3.16. But the hints makes no difference, wich version is used.
I did the migration with same version of 1.3. because, i was not sure if 1.4 will run without problems on a "old" Ubuntu 14.04. PHP 7 is only one point, systemd an another point.
So my plan was to uses a version wich will run on both systems, version 1.3.16. So i tested first if 1.3.16 will run on Ubuntu 16.04, with PHP7 and so on. And i installed all plugins, wich fits to version 1.3.16, and it works without a problem. So i migrate the data, the Database, mails and Websites to the new server, and it works, except the plugin postgrey. The other two plugins spamassassin and clamav are not clear at the moment.
But i see, that postgrey is still running on the new server. The deactivation has not really deactivate the service. Ok, its another problem with the implementation of the plugins.
At last, no support for 1.3 means no more hint or else for that in this forum. Ok, i got it.
My server runs on one leg, but it still runs. I got a bit time to think about the right backup, crash and recovery scenarios and the proper combination of os and software wich will be able to solve that.