I've resolved this issue. During user account creation time, I'm issuing the command.
/opt/zimbra/bin/zmmailbox -z -m
user@domain.com addmessage -F u "/Inbox" "/etc/scripts/introductory"
Where introductory is a folder containing the message in MBOX format.
This allows me to populate the users account with an email message of my construction as the first message in the account for a new user.