The BSD mailbox format has, according to Jamie Zawinski, never been formally documented (see his anti-content-length rant), and does not have, as far as I know, any formal identifiers that may be used as metadata for identifying a file to applications as being a BSD mailbox, except for the fact it has the 4-byte magic number 0x46726f6d. I'd like to know if there any conventions for identifying these mailbox files.
So far I have found:
- the file extension
.mboxused on some sites, supported with local scripting, but because I haven't seen this used in any widely used applications, I think this doesn't count as a convention - by contrast, RFC *822 messages are often given the .eml extension, which is widely supported, including by Microsoft and Apple; and
- no MIME type for it: it is not in the list of IANA types, although there may be informal extended types for it.
Note that I am asking for a way of communicating that a file is a BSD mailbox file, not a recogniser for them. And note also that a recogniser for BSD mailbox files that is not sure to reject mailbox files that use the Content-Length encoding is a dangerous thing, for the reasons Jamie gives in the link above: the same file might represent completely different sequences of email messages under the two schemes.