To prevent server storage DoS, there should be per-user limits to the disk space allocated to individual users. While having such a limit might not be practical, it would already be great to have a per-user limit on the number of offline messages and MAM messages.
I'm not sure which behaviour should ensue when the storage is full (sending an error sounds reasonable for mod_offline, but mod_mam should maybe just kill the oldest messages).
I'd like to see this addition as well. I've witnessed a DoS where one user received over 17GB in offline messages.
Decided to go with a limit on the number of items in the store, as this is the easiest to implement across storage backends. It doesn't have to be 100% accurate.
titleDisk space limits for mod_offline and mod_mam