Here is the specific error message text:
Blacklisted by Internal Reputation Service.
X-Authed-Username: Y25zQGh1Z2hlcy5uZXQ=
X_CMAE_Category: 0,0 Undefined,Undefined
X-CNFS-Analysis: v=2.1 cv=ZsNNU4PG c=1 sm=0 tr=0 a=bY+bM9/mKJztUtGSkaJH+w==:117 a=L9H7d07YOLsA:10 a=9cW_t1CCXrUA:10 a=s5jvgZ67dGcA:10 a=FKkrIqjQGGEA:10 a=FsKEtdHKdvMA:10 a=Kbs2a1J9zssA:10 a=o4bSt8P714vCxwCN2FUA:9 a=QEXdDO2ut3YA:10 a=no8qZEg_wP8yZpiHhfMA:9 a=_W_S_7VecoQA:10
X-CM-Score: 0
X-Scanned-by: Cloudmark Authority Engine
Received-SPF: softfail (smtp01.hughes.cmh.synacor.com: transitional domain hughes.net does not designate 10.33.66.6 as permitted sender)
Received: from [10.33.66.6] ([10.33.66.6:48367] helo=md13.hughes.cmh.synacor.com)
(ecelerity 2.2.3.49 r(42060/42061)) with ESMTP
id 39/DA-03533-BE4682E5; Wed, 22 Jan 2020 10:06:19 -0500
I know someone has suggested that it is the Client BUT ... seems highly unlikely. I have another email account, from another domain, in the same client with exactly the same settings and everything works fine for that one. And the client (emClient) has a 'diagnose and troubleshoot' function and it validates connectivity for the working domain and shows it failed for the hughesnet domain. Everything the same. Having said that, I am working with both the client developer (they are slow to respond) and with HughesNet to try to sort this out.
Remains baffling unless it has something to do with this blacklist thing.