Hi,
If by mailbox you mean a whole user mailbox, then the action should be logged in the Administrative logs (I don't remember if you have to set the log as Normal or Detailed).
It looks like:
[2019.11.02] 21:48:23.879 [ADMIN_USER_IP] User postmaster@domain.tld calling delete user, user: jason
[2019.11.02] 21:48:24.082 [ADMIN_USER_IP] User postmaster successfully deleted user jason
Sébastien Riccio
System & Network Admin
https://swisscenter.com