Mail Subject become SPAM

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.

I wonder if you are hitting a limit on the number of characters that are allowed in a subject. I have seen various lengths talked about, 78, 130, 255, 998.