Why a milter archiver? If you are required to also log source and destination IPs and BCCs (additional rcpt to's not in a CC field communicated at the SMTP protocol level), then the only option is do this in the MTA, or in a milter application that has hooks in the MTA process (which is exactly why milter was invented). It uses a simple regex file that can be used with several *source target-mailbox rules to define what gets mirrored where. The archiver tries to be as quiet as possible. The possible methods to archive are SMTP, sendmail inject / Postfix inject, IMAP delivery, and milter-add recipient. It was build as a replacement for a Postfix BCC setup: (sender_bcc_maps = regexp:/etc/postfix/archive; recipient_bcc_maps = regexp:/etc/postfix/archive).
Rumble is a mail server suite for SMTP (ESMTPSA), HTTP, POP3, and IMAP4v1 with an extensive C/C++ and Lua API for scripting. It comes with support for SSL/TLS, SQLite, and MySQL and has a Web-based administration and Web mail feature. Additional included modules feature greylisting, DNS blacklisting, SpamAssassin support, and dynamic performance scaling as well as a multithreaded structure that allows for clustering of multiple servers for one or all domains. With the Lua API you can create your own new server mailing lists, CRM software, e-marketing, and other extensions within minutes.
Proto Balance Mail is an enterprise SMTP cluster solution that supports distribution of email accounts. It scales up to 1,000,000 mailboxes apportioned over up to 125 backend mail servers (8000 mailboxes per server). No NFS or SAN is required. SOA is configurable with SOAP/XML. Anti-spam settings can be set per-user. Grey-listing is supported. Mal-ware is automatically detected and infected client PCs are automatically black-listed. POP load balancing is done. SMTP AUTH is supported. There is a Web-based management interface. Spam blocking is done by on-the-fly connection behavior analysis. It handles up to 10,000 concurrent SMTP connections. Streamlined CRM integration is done with HTTP+XML posts. Email-alias lists, forwarding, and out-of-office auto-reply are supported.