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).
Exim4U is a Web-based user interface and integrated software configuration for the Exim message transfer agent (MTA). Using any modern Web browser, you can use it to manage Exim email domains, user accounts, aliases, forwarders, and much more. Exim4U is a derivative work of the Virtual Exim (Vexim) project.