today I saw that the message retrieval wasn't working again. The POP retrieval log showed only "Starting processing for ...".
Looking at the opened TCP ports I could see that SmarterMail didn't close a POP3s connection to the mail provider (in the last both cases it was Microsoft). In the POP retrieval log you can see:
18:37:32 [...] [...] Connection to 220.127.116.11 succeeded
18:37:32 [...] [...] CMD: USER <...>
18:37:33 [...] [...] CMD: PASS XXXX
18:37:37 [...] [...] CMD: STAT
So there's no QUIT entry in the log. After closing this TCP connection SmarterMail still isn't able to do POP3 retrievals. I have to restart the SmarterMail service and then the retrieval is working again.
I don't believe there's an issue related to Windows 10 because the TCP connection is established. But it wouldn't be closed sometimes. I can also say that there's no relationship to the hourly backup because this happens to full hour times, i. e. 17:00, 18:00 and so on.
Who should close the connection? Should SmarterMail close it? Why this doesn't happen sometimes?