Sending e-mail with attachment to gmail never gets received, using phpmailer - php

Question

Something weird is going on when i'm trying to send an e-mail with attachment to gmail, using phpmailer.
Sometimes the email received after a delay of 5 minutes and sometimes it is never received. From phpmailer debugger i get that the email is actually sended, so i assume it has something to do with the attachment getting blocked by gmail.
The attachment is a zip file containing only .jpg and .pdf files.
If i remove the addAttachment('path/to/file') everything works fine.
The weird thing is that sometimes, even with the attachment, the email received immediately and other times the email never gets received.
Here is the code
$mail = new PHPMailer();
$mail->CharSet = 'UTF-8';
$mail->isSMTP();
$mail->SMTPDebug = 0;
$mail->Debugoutput = 'html';
$mail->Host = 'my_host';
$mail->Port = 'my_port';
$mail->SMTPAuth = true;
$mail->Username = 'my_username#domain.com';
$mail->Password = 'my_password';
$mail->setFrom('my_username#domain.com', 'my_username#domain.com');
$mail->addAddress('test#gmail.com', 'test#gmail.com');
$mail->Subject = $subject;
$mail->msgHTML($msg);
$mail->AltBody = $subject;
$mail->addAttachment($zipfilename);
if($mail->send()){
echo "ok";
}else{
echo "error";
}
I don't have any clue why this is happening so any help will be appreciated.
EDIT:
I found out that with two changes everything works fine.
The changes are the setting
$mail->SMTPSecure = 'tls'
and i had a path like this ./path/to/file and changed it to 'path/to/file'.