message-strip-special-text-properties

message-strip-special-text-properties is a variable defined in `message.el'.
Its value is

t

Documentation:
Strip special properties from the message buffer.

Emacs has a number of special text properties which can break message
composing in various ways. If this option is set, message will strip
these properties from the message composition buffer. However, some
packages requires these properties to be present in order to work.
If you use one of these packages, turn this option off, and hope the
message composition doesn't break too bad.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.