I recently set up an internal Exchange 2019 server for a few users that we mainly use for testing. There are only 5 accounts on there, and in one case I imported the users entire existing email backup from PST file. This backup was around 10GB, although it didn't actually import fully as it hit the "TooManyLargeItems" error. I wasn't too fussed about this as I have a PST copy of each users mailbox as an offline archive.
A few days later I enabled the online archive for this user, using all defaults of >2 years on a standalone Archives database. After this I noticed that the Exchange drive was filling up quickly. Within a few days it had grown to 100GB of the 300GB I allocated it. Eventually I realised that it was the Archive database using excess space, and found that the archive for this user is just constantly growing. After a few days it was at 50GB -
Name FolderSize ItemsInFolder
---- ---------- -------------
Top of Information Store 0 B (0 bytes) 0
Calendar 124.1 MB (130,100,547 bytes) 10241
Deleted Items 29.06 KB (29,756 bytes) 3
ExternalContacts 0 B (0 bytes) 0
Files 0 B (0 bytes) 0
Inbox 22.47 GB (24,125,848,037 bytes) 193193
Sent Items 27.09 GB (29,090,235,340 bytes) 224726
Sync Issues 0 B (0 bytes) 0
Conflicts 379.5 MB (397,940,955 bytes) 5483
Recoverable Items 0 B (0 bytes) 0
Calendar Logging 136.1 MB (142,698,692 bytes) 20681
Deletions 2.763 GB (2,967,210,171 bytes) 23899
Purges 0 B (0 bytes) 0
Versions 0 B (0 bytes) 0
Almost quarter of a million items in Sent and 200k in the Inbox....
This did not seem to stop and I have now been forced to simply disable the archive for this user.
Does anyone have any insight into how a mailbox that should only have <=10GB of email in it total can create a runaway archive like this, or what other methods I could use to understand why it happens?