How to control alignment of mail header?

I encounter a situation, when I send a email which email subject more than 80 char, sometimes the email subject will be insert a 'tab' char. I check the invalid email's mail header, they are aligned, if a mail header entry more than 80 char, it will be wrap to a new line, and insert a 'tab'. I don't know which configuration control it, javaMail setting? STMP setting?