The `default_vsz_limit` is the maximum amount of virtual memory that can be allocated. It should be set *reasonably high* to avoid allocation issues with larger mailboxes. We're setting it to 1/3 of the total available memory (physical mem + swap) to be sure.

See here for discussion:
- https://www.dovecot.org/list/dovecot/2012-August/137569.html
- https://www.dovecot.org/list/dovecot/2011-December/132455.html