I am using PHPMailer to send a basic form to an email address but I get the following error:

SMTP Error: Could not authenticate. Message could not be sent. Mailer Error: SMTP Error: Could not authenticate. SMTP server error: VXNlcm5hbWU6

The weird thing is that if I try to send it again, IT WORKS! Every time I submit the form after that first error it works. But if I leave it for a few minutes and then try again I get the same error again.

The username and password have to be right as sometimes it works fine. I even created the following (very basic) script just to test it and I got the same result

<?php

require("phpmailer/class.phpmailer.php");

$mail = new PHPMailer();

$mail->IsSMTP();
$mail->Host = "smtp.host.com";
$mail->SMTPAuth = true;
$mail->Username = "[email protected]";
$mail->Password = "password";

$mail->From = "[email protected]";
$mail->FromName = "From Name";
$mail->AddAddress("[email protected]");
$mail->AddReplyTo("[email protected]");

$mail->IsHTML(true);  

$mail->Subject = "Here is the subject";
$mail->Body    = "This is the HTML message body <b>in bold!</b>";
$mail->AltBody = "This is the body in plain text for non-HTML mail clients";

if(!$mail->Send())
{
   echo "Message could not be sent. <p>";
   echo "Mailer Error: " . $mail->ErrorInfo;
   exit;
}

echo "Message has been sent";

?>

I don't think this is relevant, but I just changed my hosting to a Linux shared server. Any idea why this is happening?

Thanks!

***UPDATED 02/06/2012 I've been doing some tests. The results:

I tested the script in an IIS server and it worked fine. The error seems to happen only in the Linux server.

Also, if I use the gmail mail server it works fine in both, IIS and Linux.

Could it be a problem with the configuration of my Linux server??