Microsoft touts Exchange 2007's ability to support large mailboxes because of the larger memory model made possible by the Windows x64 platform. Having large mailboxes will certainly please the human pack rats that populate many Exchange servers, but the growth in mailbox size does pose some management challenges. Exchange 2007 will make managing large mailboxes easier, but what can you do until then?
The Super Size Phenomenon
Mailbox sizes have steadily increased from the 25MB quota that was commonly
allocated for Exchange Server 4.0 deployments to today's generous quotas of
250MB?500MB. Users argue that the time they spend keeping their mailboxes within
a strict company-imposed size limit, coupled with the low cost of storage, makes
it more cost effective for companies to simply allow larger mailboxes. The result
is that many large mailboxes are in use today, some of which approach 20GB.
Of course, users like large mailboxes and have a tendency to use all the space you give them. But regrettably, some features of email clients, such as the Reply All option and the way Outlook lets you include previous messages in a mail thread, cause users to fill their mailboxes even faster than necessary. Using Microsoft Word instead of a simpler editor also contributes to large mailboxes, as does using graphics in auto signatures on every message. . . .

