Where it accumulates
IMAP keeps everything on the server by default. Sent items with attachments, and shared mailboxes nobody prunes, are the usual bulk.
Practical controls
Worth setting early:
- Per-mailbox quotas
- Automatic trash and spam expiry
- Archiving old mail off-server
- Alerts before the disk fills
Watch the catch-all
A catch-all address collects every misdirected message and every spam attempt at your domain. It is convenient for a month and a storage problem for a year.