List Mailboxes and Size
You can find list of mailboxes and space taken by each of them using following command/script (source):
if [ -d /var/qmail/mailnames ]; then echo -ne "\n\n=== MAILBOXES ===\n"; cd /var/qmail/mailnames && TMB=$(du -ks */* 2>/dev/null | sort -nr | cut -f2); if [ -n "$TMB" ]; then echo "$TMB" | xargs du -sh; fi; echo "[`find . -mindepth 2 -maxdepth 2 -type d | wc -l` Mailboxes - Total `du -hs | cut -f1`]"; fi;
It will generate output like:
=== MAILBOXES ===
3.6M example.com/bob
3.3M rtcamp.com/joe
[2 Mailboxes - Total 3.9M]
List Mailboxes and Passwords
Plesk has builtin command for this. Simply run:
/usr/local/psa/admin/bin/mail_auth_view
And you will see something like:
+------------------ ---+-----+---------------+
| address |flags| password |
+----------------------+-----+---------------+
| bob@example.com | | hello |
| joe@rtcamp.com | | world |
+----------------------+-----+---------------+
Flags
A - account disabled
D - domain disabled
E - password encrypted
Mail Queue
Pleak has another built-in command to inspect mailqueue.
Running /usr/local/psa/admin/bin/mailqueuemng will show useful help about different parameters.
References
Check this for awesome collection of Plesk commands and cheats.