dovecot - Jun 2016 - Advice needed: SMTP/LMTP or IMAP for internal message delivery

I?ve implemented ?next day? delivery this week by taking messages submitted
through Postfix, queuing the messages in a MySQL database, and sending them out
for delivery through another Postfix instance/Dovecot LMTP proxy with final
delivery using the destination mail servers LMTP service. Delivery is sent to a
?next day? mail server which is sync?d to the recipient?s mail server using
?doveadm sync -A tcp:next-day-mail:12345? on each mail server in the early
morning.

This all appears to be working well. I do have a need to deliver these messages
into specific mail folders in the recipient?s mailbox and I was planning on
using global Sieve scripts running on the ?next day? mail server to place the
messages in the proper folder (not all messages are delivered to the INBOX
folder).

I am using a PHP script to move the messages from Postfix to the MySQL database
and then later sending them via SMTP to the internal Postfix instance for
delivery to the ?next day? mail server. I started investing how to code the
Sieve scripts today and it occurred to me that I could greatly simplify message
delivery by using IMAP to deliver the messages to the ?next day? mail server.
The PHP script would be able to deliver the exact message that should be stored
to the proper folders setting the \Seen flags appropriately.

My backend ?next day? mail server already supports Master password login so I
figure the PHP script should be able to login via IMAP to access the folders in
the recipient mailboxes. Also, the PHP script could do other mailbox maintenance
tasks when it connects to the user?s mailbox such as purge folders by age,
message count, etc. The PHP script could also retrieve info about the current
state of the user?s mailbox folders (like the date of the oldest unread message,
how many messages have been read in the last week or so, etc) and store this
data in the MySQL DB.

I?m looking for any advice on whether to scrap the current plan of deploying
internal Postfix SMTP/Dovecot LMTP proxy/Dovecot LMTP/Sieve script for ?next
day? mail delivery and just write PHP script to access Dovecot IMAP (direct to
the ?next day? mail server or to the user?s mail server for ?immediate?
delivery).

This would allow me to drop using Dovecot LMTP at all. Postfix SMTP would only
be configured to invoke a PHP script to deliver messages to the database (which
is already implemented). Postfix would still use Dovecot IMAP authentication,
but only Dovecot IMAP service would need to be ?highly available?.

Any opinions? Should I dump LMTP?

Kevin