The GWIA has four services of SMTP, POP, IMAP and LDAP.
The SMTP service when sending and receiving messages uses the path to the MTA that owns the gateway via the UNC and MTP path. Its a store and forward transport of the messages.
The POP and IMAP services use the C/S 1677 tcp path which goes direct IP to the various GWPOAs where the users mailbox resides. Its a real time live connection to the users mailbox. The path through the MTA is not used in this case.
The LDAP service uses the UNC path to the MTA that owns the gateway via the UNC path. It does a look-up within the DB file to satisfy the LDAP query.
There you go.