SOAP method mail_user_update limits quota to about 2GB
When using the SOAP method mail_user_update with a quota bigger than 2147483647 (2^31 - 1) bytes, the quota is limited to 2147483647 (2^31 - 1) bytes. So for example changing the move_junk field via SOAP will set the quota of the user to ~2GB if it has been set bigger before.