I need to recover an email that another user on my network sent a few days ago. I got the email ID using the grep command on /var/log/mail.log, but I don't know what to do right now with this.
How can I use the ID to view the contents of an email?
You can't get the contents of the email through your logs. Only when it was processed, and if it was sent of to some local mail transport or relayed. If the email is inbound and you have some mail transport setup (e.g. cyrus/pop3/imap), then you may want to look in the mailbox for the email ID. Otherwise, if the mail was to be relayed elsewhere, and was deferred or ran into a temporary error, you can check in the postfix queues:
postqueue -p
should list all email in the postfix queues. You can then look for the email itself in the queue with the given queue ID, which should probably be in /var/spool/postfix