Email clients like Outlook or Thunderbird can be automatically configured to connect to the mail server on the correct ports and the settings for the encryption can be adjusted (especially with Outlook you can easily slip).
There is a new plugin for ISPConfig: ISPConfig Automail
This allows a central website to be set up, which then automatically generates the corresponding XML files.
Especially a wrong hostname like kunde.domain.de leads to unnecessary support requests, why and why the certificate is invalid.
With ISPConfig Automail the email clients can be set up automatically for any number of mail servers and mail domains.
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