Mail Subject become SPAM

[newphpbees]

I have code for mail subject:

$Mail->Subject = 'Preview/' . $this->csvUploadPaymentDetails[0]['csvfilename'].'/'.date('d-m-Y').'/'.$this->csvUploadPaymentDetails[0]['week_num']; 

The problem is the mail was become SPAM, I got this message
Mail delivery failed: returning message to sender. How can I prevent my email become not SPAM?

When my subject is only this: $Mail->Subject = 'Preview'; it works, but I need my subject is

 $Mail->Subject = 'Preview/' . $this->csvUploadPaymentDetails[0]['csvfilename'].'/'.date('d-m-Y').'/'.$this->csvUploadPaymentDetails[0]['week_num']; 

, because I need to have filename,date, and weekno in my subject.

It's my first time to encounter this error.

Thank you..any help is highly appreciated.