@paolo 50,000 is way too many bounces for phplist to process. If this number of bounces has built-up over a long period of time then you should just delete them all from the mailbox, or maybe leave the most recent few hundred.
Then phplist will be able to deal with bounces so long as you run process bounces regularly. You should also actively monitor the bounces so that the subscribers are removed or unconfirmed or blacklisted.
Also, you should not allow the number of bounces to build-up in the phplist database, so you need to periodically delete old ones or use advanced bounce rules.
There is a setting in the config file that controls whether to delete emails from the mailbox. The default is to delete, but you might have changed that.