3 Protocol Details
The higher layer is the Directory Replication Service (DRS) Remote Protocol (as specified in [MS-DRSR]). The lower layer is the SMTP MTA delivery function.
The DRS Protocol Extensions for SMTP serializes a DRS message and encloses that message in its own message envelope, which is called the DRS Protocol Extensions for SMTP frame. The DRS Protocol Extensions for SMTP first inserts the extension frame into a MIME attachment, then inserts the MIME attachment into an SMTP message, and finally gives the SMTP message to the lower-layer SMTP MTA for delivery to the recipient.