How do I update Postfix .db files after removing unwanted emails?

I know almost nothing about setting up email, and have inherited an old Ubuntu 6.10 server with live sites and active email. I'm concerned about upgrading Ubuntu without checking this first. It's running:

• dovecot --1.0.15
• mail_version = 2.5.5 (postfix)

I just removed old emails from:

• etc/dovecot/passdb
• etc/dovecot/userdb

and from

• etc/postfix/virtual
• etc/postfix/vmailbox

then ran doveadm reload (command not found), then postfix reload which just echoed the command, so not sure if that worked.

However, etc/postfix/virtual.db still contains the unwanted emails.

I'm aware I'm hacking badly here, so looking for pointers, as it appears one of the accounts is sending spam, so my priority task is to identify where/how - so the first step was to remove all unused emails.