title: Why does an email subject contain linefeed or carriage return characters?
body: <p>I'm making a code to check a mailbox, and forward unseen mails to another user.<br>
  But sometimes it fails with an error:</p>
  
  <blockquote>
    <p>ValueError: Header values may not contain linefeed or carriage return characters</p>
  </blockquote>
  
  <p>I checked the raw fetched data and found out that the 'Subject' value contains \r\n.<br>
  Not all mails contain, but some do.<br>
  It just appears normal in the mailbox, and I have no idea why some contain such characters.<br>
  Does it have to do with the length of the subject?<br>
  How can I deal with these situations?<br>
  Thanks :)</p>