Our Exchange 5.5 SP4 box is perpetually filling the disk
to near capacity, full of .dat files in the MTAData
directory. All of the messages sitting in there are spams
sent to non-existent mailboxes at our domain. The server
is accepting mail for these invalid boxes but not trying
to bounce it back, thus the MTAData directory is filling
the disk to capacity with undeliverable mail that just
sits there unprocessed. Every couple days we have to shut
down Exchange and manually delete hundreds of thousands
of .dat files.
Any ideas what might be wrong, or is there any way to
force the server to perform a valid-recipient check before
accepting mail? Undeliverable mail is collecting in this
directory at the rate of several gigs per week, so it's a
very serious problem for us.