Github user ijokarumawak commented on the issue:
https://github.com/apache/nifi/pull/483
@trixpan I found why I got the `Too many connection` exception.
I didn't setup SSL at ListenSMTP processor, so it was expecting plain text
connections.
But PutEmail uses `javax.net.ssl.SSLSocketFactory` by default, although
it's configurable, I didn't. Then PutEmail tries to open SSL connection while
ListenSMTP expects plain text.
Then `javax.mail.SocketFetcher` fallbacks from SSL to Plain socket, as
`mail.smtp.socketFactory.fallback` is set to
[true](https://javamail.java.net/nonav/docs/api/com/sun/mail/smtp/package-summary.html).
It opens SSL socket, and got an exception, then reopen plains socket. That's
why I see two connection attempts on SMTP server side.
I suspect there's a race condition between SMTPServer releasing the
Semaphore and PutEmail requesting 2nd connection (fallback).
After I changed `SMTP Socket Factory` to `javax.net.DefaultSocketFactory`,
the error disappeared.
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