Openexchange Server: Version 4
A multidrop mailbox is usually an ordinary POP3 (Post Office Protocol) mailbox provided by the ISP where all the messages of a complete domain are stored. The messages are fetched from the mailbox via POP3 and then distributed in the destination system. However, since the SMTP transport (Simple Mail Transfer Protocol) has already been completed at the provider's, some information has been lost, the most important of which being the recipient address, also known as Envelope Recipient Address.
An e-mail message consists of an envelope, a header, and a body. The envelope is created and transmitted from MTA (Mail Transfer Agent) to MTA. It can be compared to remarks in a rotating file. Once the document has been filed and the envelope removed, the recipient can only be determined on the basis of the content. The same occurs with e-mail messages. When the message is stored in a POP mailbox, the envelope is discarded. This is not a big problem if the message is sent from one user to another. The envelope's recipient is identical to the recipient in the mail header (To:). However, if several users are subscribed to a mailing list, the To: address reads mailinglist@domain.com for every user this message is directed to. The real recipient is transmitted through the SMTP command RCPT TO during the mail transfer. Once the message arrives to the destination server (the provider's POP3 server), this key information has been lost.
The problem also arises if the message is directed to several recipients. The envelope contains only one recipient and each recipient usually receives its own message. However, the header contains all recipients in no particular order.
In fact, the envelope information is not totally lost. Otherwise, not a single software would be able to distribute messages from multidrop mailboxes. The destination address leaves some traces in the form of Received: lines in the header, which enable to disclose the real recipient. Unfortunately, this information is not standardized, i.e., each MTA writes these lines in a different way.
Thus it appears that, when using multidrop, errors are bound to happen time and again during the mail delivery process. Therefore it is advisable to avoid the use of multidrop mailboxes whenever possible.