Github user olegz commented on the issue:
https://github.com/apache/nifi/pull/710
Digging into the source code for it looks like something is out of
compliance with POP3 protocol
```java
if (line.startsWith("+OK"))
r.ok = true;
else if (line.startsWith("-ERR"))
r.ok = false;
else
throw new IOException("Unexpected response: " + line);
int i;
```
So, it appears that the received message is not really compliant with POP3,
but will dig some more in the AM
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