Don’t worry, this code is just visible in the ‘original’ or ‘raw’ view of your email. It doesn’t mean, that your email code was changed. This code is added to your emails after it arrived to the mailbox and it is only there for creating better looking printed emails.
In order to deal with special characters (or non-8 bit encoded emails) those certain characters are added. Modern ESMTP servers are in most cases 8-bit clean, so any 8-bit byte value may be encoded with 3 characters: an equal sign followed by two hexadecimal digits representing the byte’s numeric value.
You can use this free tool to encode and decode quoted printable formats.