Error 500 on Digital Ocean while using SwiftMailer to send email

I am trying to send an email to myself from my seconday email address to my primary email address when a contact form is submitted to my website. I am using Swiftmailer to achieve this. The code works perfectly when I use it offline on my XAMPP server on my computer, but when I try to run it on my Digital Ocean Droplet running a LAMP stack, which is almost identical to my offline setup, it doesn't work and returns an error 500.

I looked into it and it turns out, SMTP is disabled to prevent spam and it can be lifted if I contacted support. After I contacted them, they told me they couldn't lift the restriction on port 25 but told me to try alternative ports and that SMTP doesn't work with Floating IPs. I removed my floating IP to help with this and I am currently trying to use port 578 with a Hotmail email address, but it still doesn't work.

What I have tried so far:

I used this command to open up the port 587 as suggested by Digital Ocean Support here.

sudo iptables -A INPUT -p tcp --dport 587 -m conntrack --ctstate NEW,ESTABLISHED -j ACCEPT
sudo iptables -A OUTPUT -p tcp --sport 587 -m conntrack --ctstate ESTABLISHED -j ACCEPT

After that, my server is able to connect to smtp.office365.com at port 587 as showed by the command below:

nc -vz smtp.office365.com 587
Connection to smtp.office365.com 587 port [tcp/submission] succeeded!

The command "ufw status" shows that port 587 is open on both ipv4 and ipv6:

To                         Action      From
--                         ------      ----
22/tcp                     LIMIT       Anywhere
80/tcp                     ALLOW       Anywhere
443/tcp                    ALLOW       Anywhere
3306/tcp                   ALLOW       Anywhere
587/tcp                    ALLOW       Anywhere
22/tcp (v6)                LIMIT       Anywhere (v6)
80/tcp (v6)                ALLOW       Anywhere (v6)
443/tcp (v6)               ALLOW       Anywhere (v6)
3306/tcp (v6)              ALLOW       Anywhere (v6)
587/tcp (v6)               ALLOW       Anywhere (v6)

But, when I try to run the contact form, it doesnt work in the end on the droplet, while it does work offline just fine and I received the emails as intended.

Is there something I am missing?

I will attach the code for my swiftmailer part below, just incase it helps identify any potential errors in the code that I seem to be missing.

include_once 'private/pass.inc.php';


//These fields usually get the data using a a form using POST method and PHP but left with placeholders here for the sake of simplicity.
$name = 'name';
$email = 'email';
$phone = 'phone';
$subject = 'subject';
$message = 'message';

// Send Email
// Create the Transport
$transport = (new Swift_SmtpTransport('smtp.office365.com', 587, 'tls'))
    ->setUsername($emailusername)
    ->setPassword($emailpassword);

// Create the Mailer using your created Transport
$mailer = new Swift_Mailer($transport);

function sendVerificationEmail($name, $email, $phone, $subject, $message)
{
    global $mailer;
    $sentfrom = "[email protected]";
    $sentto = "[email protected]";
    $body = 'Message From:' . $name . '<br>' . $email . '<br>' . $phone . '<br>' . $subject . '<br>' . $message;
    // Create a message
    $message = (new Swift_Message("A New Message From " . $name))
        ->setFrom([$sentfrom])
        ->setTo([$sentto])
        ->setBody($body, 'text/html');

    // Send the message
    $result = $mailer->send($message);
}

sendVerificationEmail($name, $email, $phone, $subject, $message);

echo "<script> window.location.assign('./contactformsuccess'); </script>";

EDIT: Added Logs

[Sun Aug 15 07:34:58.851255 2021] [php:warn] [pid 182629] [client 162.158.167.91:37848] PHP Warning: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages:\nerror:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed in /var/www/site.com/html/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/StreamBuffer.php on line 94 [Sun Aug 15 07:34:58.851487 2021] [php:error] [pid 182629] [client 162.158.167.91:37848] PHP Fatal error: Uncaught Swift_TransportException: Unable to connect with TLS encryption in /var/www/site.com/html/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php:349\nStack trace:\n#0 /var/www/site.com/html/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/AbstractSmtpTransport.php(148): Swift_Transport_EsmtpTransport->doHeloCommand()\n#1 /var/www/site.com/html/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Mailer.php(65): Swift_Transport_AbstractSmtpTransport->start()\n#2 /var/www/site.com/html/includes/cf2.inc.php(39): Swift_Mailer->send()\n#3 /var/www/site.com/html/includes/cf2.inc.php(42): sendVerificationEmail()\n#4 {main}\n thrown in /var/www/site.com/html/vendor/swiftmailer/swiftmailer/lib/classes/Swift/Transport/EsmtpTransport.php on line 349