What version are you running?
2.5.9
What's the URL of the page this enhancement relates to, if any?
N/A
Describe the enhancement and the motivation for it.
When a diff contains a large number of files, the SMTP header becomes very large very quickly.  In our case, we have had users create requests that cannot send any email because the header is larger than the system max.  (Yes, ideally diffs should not be this large.)
I would like more configuration flexibility regarding SMTP headers.  Ideally, this might involve being able to turn on/off any of the standard header values.  In the simplest case, to fix our problem, being able to turn off the 'X-ReviewBoard-Diff-For' header would be good enough.
What operating system are you using? What browser?
CentOS 7.x / FF,Chrome,etc., but browser is irrelevant here.
Please provide any additional information below.