allow InReplyTo for sending email via Mail API
In-Reply-To is a standard email header, and right now there's no way to add it aside from it as a custom InternetMessageHeader. Even so, the API complains that In-Reply-To needs to be prefixed with 'X-' which is not true.
swaroop reddy commented
it is also helpful to send in-reply-to headers back in the messages query. external programs use this to organize messages and their relationships for analysis.