SMTP Error (554): sending to remote adresses (relaying) is not allowed

[mcnesium]

When trying to send mail, no mail is sent, but I see an error popup stating:

SMTP Error (554): sending to remote adresses (relaying) is not allowed

The logfile says that:

roundcube_app | errors: <8b77a638> PHP Error: Invalid response code received from server (POST /?_task=mail&_unlock=loading1586892896811&_framed=1&_lang=de_DE&_action=send)errors: <8b77a638> SMTP Error: Failed to add recipient '[email protected]'. Refused. Sending to remote addresses (relaying) is not allowed. (Code: 554) in /var/www/html/program/lib/Roundcube/rcube.php on line 1702 (POST /?_task=mail&_unlock=loading1586892896821&_framed=1&_lang=de_DE&_action=send)172.20.0.1 - - [14/Apr/2020:19:34:56 +0000] "POST /?_task=mail&_unlock=loading1586892896821&_framed=1&_lang=de_DE HTTP/1.1" 200 979 "https://webmail.domain.net/?_task=mail&_action=compose&_id=8230826045e961043f04c8" "Mozilla/5.0 (X11; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0"

This is in Roundcube 1.4.3, installed with Docker via composer:

FROM roundcube/roundcubemail:latest

RUN set -ex; \
    apt-get update; \
    apt-get install -y --no-install-recommends \
        git \
    ; \
    \
    curl -sS https://getcomposer.org/installer | php -- --install-dir=/usr/bin --filename=composer; \                                             
    mv /usr/src/roundcubemail/composer.json-dist /usr/src/roundcubemail/composer.json; \                                                          
    \
    composer config --file=/usr/src/roundcubemail/composer.json minimum-stability dev;                                                            

RUN composer \
        --working-dir=/usr/src/roundcubemail/ \
        --prefer-dist --prefer-stable \
        --no-update --no-interaction \
        --optimize-autoloader --apcu-autoloader \
        require \
            elm/identity_smtp:=dev-master \
    ; \
    composer \
        --working-dir=/usr/src/roundcubemail/ \
        --prefer-dist \
        --no-interaction \
        --optimize-autoloader --apcu-autoloader \
        update;

What is the matter here? How can I try to fix this?

[mcnesium]

I found out that it works, when commenting the encryption of passwords here and putting just $smtpSettings['smtp_pass']; instead of the decrypting here and then re-enter the passwords in the identity settings (passwords are then stored unencrypted to the database!) So there must be something wrong with the password en- or decryption. Who can help?