It has been pointed out to me that the email-arch document uses the words for
"MS" inconsistently, varying between "message store" and "mail store".
On check their use in the IETF context -- even within a single email working
group -- I find that indeed, I'm not the only one varying between the terms.
Although the document purports to describe Internet *mail*, I am going to
define MS to mean "message store", unless enough of you object.