Github user ijokarumawak commented on the issue:
https://github.com/apache/nifi/pull/483
Looked at the SMTP server side, org.subethamail.smtp.server.SMTPServer to
investigate how it uses maxConnections. It's used as a Semaphore. With
successful transport, there's a QUIT message and server release the Semaphore.
So next client can send message:
```
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Client:
MAIL FROM:<n...@example.com>
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Server: 250
Ok
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Client:
RCPT TO:<n...@example.com>
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Server: 250
Ok
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Client: DATA
2016-07-11 12:09:58,794 D [org.subethamail.smtp.server.Session] Server: 354
End data with <CR><LF>.<CR><LF>
2016-07-11 12:09:58,795 D [org.subethamail.smtp.server.Session] Server: 250
Ok
2016-07-11 12:09:58,795 D [org.subethamail.smtp.server.Session] Client: QUIT
2016-07-11 12:09:58,795 D [org.subethamail.smtp.server.Session] Server: 221
Bye
```
But I noticed that NiFi tries to open new connection before the previous
one is closed:
```
2016-07-11 12:05:16,151 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57774] SMTP connection from
localhost/127.0.0.1, new connection count: 1
2016-07-11 12:05:16,151 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57774] Server: 220 127.0.0.1
ESMTP Apache NiFi
2016-07-11 12:05:16,152 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57774] no more lines from client
2016-07-11 12:05:16,152 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57775] SMTP connection from
localhost/127.0.0.1, new connection count: 2
2016-07-11 12:05:16,152 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57775] SMTP Too many
connections!
2016-07-11 12:05:16,152 D
[org.subethamail.smtp.server.Session-/127.0.0.1:57775] Server: 421 Too many
connections, try again later
```
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