User_external: Login fails; config error?

[husky]

Hi,
I recently set up a new NC host with NC 18 alongside a dockerized mail-stack. Now, for ease of maintenance I’d like to allow my mail users to login using their mail credentials - the user_external app seems to be just the appropriate extension.

So, I installed the app and tried to configure it according to the documentation, but so far I can’t log in any of my mail users.

My setup:

• Debian Buster
• NGinx with a few named virtual hosts, facilitating LetsEncrypt-certificates and SNI (all running well)
• Mailcow, a dockerized mail stack. Ports 25, 110, 143, 993, 995 are reachable via localhost address

the relevant part of my config.php:

‘user_backends’ =>
array (
‘class’ => ‘OC_User_IMAP’,
‘arguments’ =>
array (
0 => ‘127.0.0.1’,
1 => 143,
2 => ‘tls’,
3 => ‘example.comf’,
4 => true,
5 => false,
),
),

Changing 127.0.0.1 to the FQDN or to {127.0.0.1} (seen that somewhere) doesn’t improve my login success.

Any hints anyone? I’d appreciate