How to get SMTP mail driver working on Forge+LetsEncrypt?

Hi,

I've been trying to get the SMTP mail driver for Laravel to work on a Forge server that uses LetsEncrypt. Problem is, that I get this error message when trying to send an e-mail via artisan tinker:

"PHP warning:  stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed in /home/forge/intranet.trainingenopmaat.nl/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php on line 95"

Anyone have a suggestion on how to get the SMTP mail driver working on a Forge server that uses Let's encrypt?

Testing steps

Code I use in artisan's tinker functionality to test testing:

Mail::raw('This is a test', function ($message) 
{
    $message->to('{snip primary mail address}');
    $message->cc('{snip secundary mail address}');
    $message->subject('This is a test');
});

My basic settings: MAIL_DRIVER=SMTP, MAIL_ENCRYPTION=TLS

What I've tried so far

Editing PHP Ini values for certs

According to Google, this has to do with SSL certificate configuration not being correct. See http://stackoverflow.com/questions/34515120/php-verify-failed-with-letsencrypt

Following the above URL I've tried adding the URL the Forge control panel provided (which points to ".../server.crt") to the "curl.cainfo" and "openssl.cafile" keys in the PHP-FPM and PHP-CLI ini files to test if that works, but it doesn't.

For one, if I do that and run php artisan tinker and try the mail function, I get:

PHP warning:  failed loading cafile stream: `{snip}/server.crt' in /home/forge/{snip}/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php on line 95 (according to the PHP source code this has to do with not being able to open the file, so I reckoned it was maybe a permissions issue)

Secondly, if I run sudo php artisan tinker, I get the same old verify certificate error again:

PHP warning:  stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:ssl3_get_server_certificate:certificate verify failed in /home/forge/{snip}/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php on line 95

Also, my entire application fails when I set this, because it'll die with the following error:

[2016-06-15 14:37:04] production.ERROR: exception 'GuzzleHttp\Exception\RequestException' with message 'cURL error 77: error setting certificate verify locations:
  CAfile: /etc/nginx/ssl/{snip}server.crt
  CApath: /etc/ssl/certs (see http://curl.haxx.se/libcurl/c/libcurl-errors.html)' in /home/forge/{snip}/vendor/guzzlehttp/guzzle/src/Handler/CurlFactory.php:187

(I do not know what this means - googling determines not having the ca-certificates Ubuntu package installed could be cause of this bug, but I have it installed. I haven't done any further checking yet)