Thanks! That's an interesting read. We were actually wondering what was the maximum number of items in References field but couldn't find any. This RFC does give an answer[1]!
[1] http://tools.ietf.org/html/rfc5322#page-25
EDIT: fixed typo
[1] http://tools.ietf.org/html/rfc5322#page-25
EDIT: fixed typo
[1] http://datatracker.ietf.org/doc/rfc5322/