Owncloud caldav: appointment request not sent

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Dear forum,
I am using miab for 3 month now.
I was confused about not being able to send appointment request through caldav (thunderbird, macos).

I found out that the default mail settings oft owncloud are not functional: Requests are only sent if the settings are changed to SMTP

Here is my working config.php (/home/user-data/owncloud/config.php)

  'mail_from_address' => 'nextcloud',
  'mail_smtpmode' => 'smtp',
  'mail_sendmailmode' => 'smtp',
  'mail_domain' => 'XXX',
  'mail_smtphost' => 'XXX.de',
  'mail_smtpport' => '465',
  'mail_smtpauth' => 1,
  'mail_smtpname' => 'nextcloud@XXX.de',
  'mail_smtppassword' => 'XXX',

Furthermore for internal users e-mail adresses have to be added manually to nextcloud otherwise appointment requests are ignored by the system.

Would you please fix that problems!

Thanks in advance!

Regards,

Fabian