FTP error cannot connect

(Sorry forgot to change the subject line first send.)

We have a process that sends files via FTP and been trying to FTP to an outside vendor, and we have been getting this message:
Cannot connect to host (ip address) at address . Try again later.

This only happens when connecting to the FTP server from the iSeries.
Other systems (not iSeries) such as a PC can connect.
Both our operations manager and the people at the vendor's site seem unable to determine what to do.

FTP does work going from our iSeries to other locations. It's this one vendor's system where we have an issue.

Now, here's the weird part:
We're working on the implementation. The vendor has two systems, a test and a production system.
The production system is where we first encountered the problem. FTP was working fine on the test system.
While trying to find the problem, and I was able to get a connection by doing FTP 'the_url.com'
That is, until last Tuesday. Then, all of a sudden, the test system also got the "cannot connect... try again later" error.

The vendor says nothing changed with the test server on their end.
The operations manager says nothing changed on our end either.
He's put in some time trying to research the problem.

I don't know if the problem is happening on the vendor's site, on our iSeries, or in the firewalls in between.
I don't even know how to figure out where the problem is happening.

We are able to ping both the test site and the production from the iSeries.
I hope someone can steer us in the right direction on what to do.

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