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I have been checking the logs, the only logs that return some interesting stuff and the "mailgate.log" log which is in the "/ var / www / glpi / files / _log /" directory.
I have no error in the "php-errors.log" and "sql-errors.log" logs which concern the collector.
Here is what the log says:
Invalid message: Malformed header detected <br/> 2020-10-01 15:10:06 [@SERVER]
Invalid message: Malformed header detected <br/> 2020-10-01 15:10:06 [@SERVER]
Invalid message: Malformed header detected <br/> 2020-10-01 15:10:06 [@SERVER]
Obviously the mailgate tells me that the mail header is badly formatted (this only since the migration to 9.5.1).
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Regarding the configuration of the collector:
Server type: onpremise (internal) "Kerio Connect"
Connection type: POP
TLS: NO-TLS
Authentication: YES
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I see a lot of people having problems with the collector since the update to version 9.4, see 9.5.5, but no log return, if that might give some people a clue.
Looking forward to your feedback, thank you.