I am having the same problem too.
Fresh install of 5.2.14. Installation method was with git, but the command was slightly different.
git clone https://github.com/invoiceninja/invoiceninja.git --depth 1 --branch v5-stable --single-branch
Running on a shared host. PHP 7.3.27. Composer 2.0.6. Trying to send using SMTP with Mailgun SMTP credentials. Fails for both encryption types and all ports Mailgun SMTP supports. Sendmail and Mail methods are failing also with the same error.
I don’t really know if the server has all the required PHP modules. I presumed the installation had preflight checks that would identify anything missing.
/opt/cpanel/ea-php73/root/etc/php.d/20-bcmath.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-bz2.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-calendar.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-ctype.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-curl.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-dba.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-dom.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-enchant.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-exif.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-fileinfo.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-ftp.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-gd.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-gettext.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-gmp.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-iconv.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-imap.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-intl.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-json.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-ldap.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-mbstring.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-mysqlnd.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-odbc.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-pdo.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-pgsql.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-phar.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-posix.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-pspell.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-shmop.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-simplexml.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-soap.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-sockets.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-sqlite3.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-sysvmsg.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-sysvsem.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-sysvshm.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-tokenizer.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-xml.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-xmlwriter.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-xsl.ini,
/opt/cpanel/ea-php73/root/etc/php.d/20-zip.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-mysqli.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-pdo_mysql.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-pdo_odbc.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-pdo_pgsql.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-pdo_sqlite.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-wddx.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-xmlreader.ini,
/opt/cpanel/ea-php73/root/etc/php.d/30-xmlrpc.ini,
/opt/cpanel/ea-php73/root/etc/php.d/zzzzzzz-pecl.ini
I should add that laravel.log is empty