Swiftmailer Error: Recipient Rejected - php

Question

I am trying to get Swiftmailer to work but keep getting "recipient rejected" messages.What causes the "recipient rejected" response? Is this a response from the gmail server (gmail sensing this is SPAM)? Is there a way to overcome this? Otherwise, I will abandon Swiftmailer and try PHPMailer. I've had success with PEAR packages in the past but got tired of the configuration and was going to try Swiftmailer first...it shouldn't be this difficult right?
My Configuration:
- PHP 5.5.6
- Swiftmailer 5.0.3 (with Logger Plugin)
- Hosting by GoDaddy (yeah...I know)
Goal: Use Swiftmailer to send SMTP message from admin#mydomain.org to legit#gmail.com
(obviously the email addresses are placeholders for actual addresses)
Here is the PHP code: (same as example in Swiftmailer documentation)
<?php
require_once 'lib/swift_required.php';
$transport = Swift_SmtpTransport::newInstance('smtp.mydomain.org', 25)
->setUsername('admin#mydomain.org')
->setPassword('mypassword')
;
$mailer = Swift_Mailer::newInstance($transport);
$logger = new Swift_Plugins_Loggers_EchoLogger();
$mailer->registerPlugin(new Swift_Plugins_LoggerPlugin($logger));
$message = Swift_Message::newInstance('Wonderful Subject')
->setFrom(array('admin#mydomain.org' => 'admin'))
->setTo(array('legit#gmail.com' => 'recipient'))
->setBody('Here is the message itself')
;
$result = $mailer->send($message);
echo $logger->dump();
?>
Here is the relevant logger info:
...
++ Swift_SmtpTransport started
...
>> RCPT TO: <legit#gmail.com>
<< 550 5.1.1 <legit#gmail.com> recipient rejected
!! Expected response code 250/251/252 but got code "550", with message "550 5.1.1 <legit#gmail.com> recipient rejected "
>> RSET
<< 250 2.0.0 OK
++ Stopping Swift_SmtpTransport
>> QUIT

Answer 1

Not a "real" answer, but too long for the comment box:
Is this a response from the gmail server
Yes.
recipient rejected means that, somehow, the recipient (that is, the address you're sending mail to, legit#gmail.com`) is "rejected".
This can have a number of causes
Email address doesn't exist (anymore) (quite probable)
User mailbox is full (quite probable)
The account is blocked by gmail (quite probable)
the user set a filter to explicitly reject emails from you (not sure if this can be done using gmail, but it's technical possible).
Your message is considered spam (unlikely, you would probably get a different message)
... many others...
It's somewhat unfortunate that gmail doesn't provide more info as to the cause. I can't find any documentation on gmail's error codes (550 is a generic "it failed" code), and usually these sort of messages are more verbose ...
What you could try:
Make very sure the email exists and is valid. Use a different gmail address (ie. one accessible by you) to be sure.
Check if you're blacklisted, for example using: http://mxtoolbox.com/blacklists.aspx
Try sending mail from smtp.mydomain.org, for example with the commandline mail utility (if it's a UNIX box), if this doesn't work, you know the problem is at the mail server, if it does work, you know the problem is at Swift mailer.