While sending mail through php I am getting below error-

501 Syntax error - line too long 
The following SMTP error was encountered: 501 Syntax error - line too long 
Unable to send email using PHP SMTP. Your server might not be configured to send mail using this method.

As above message clearly explains the issue but I don't want to reduce the content of html template. Any idea where I need to change settings ?

I am using these details-

'smtp_host' => 'smtp.1and1.com',
                'smtp_port' => '587',