MimeMessage.Bodyis a tree structure like MIME, so you need to go to the MimePartone containing the content you want to change.
, /* MimePart, MimePart, TextPart, .Text ( ).
, MIME , , : http://www.mimekit.org/docs/html/WorkingWithMessages.htm
:
var part = message.BodyParts.OfType<TextPart> ().FirstOrDefault ();
part.Text = part.Text.Replace ("x", "y");
, , /*, , , .