The RFC you quote does not define the mbox format, but the mbox media format. They are different things. THe mbox file format despite being refered to in the RFC you quote has never been defined in any RFC. and RFC 4155 actualy says "The mbox database format is not documented in an authoritative specification, but instead exists as a well-known output format that is anecdotally documented, or which is only authoritatively documented for a specific platform or tool."

Thunderbird uses the RD sub format of mbox. That is a legacy from decisioons made by the Netscape development team in the 1990s. The MBOXRD sub foirmat named for Rahul Dhesi who developed the format in 1995 offers reverse quoting of from: line. The version used in Mozilla MUA based mail clients is actually a further variant on the MBOXRD format in so much as the from line is defined in greater details.

Thunderbird use of the MBOX formated files will probably change in the next version as work has seriously started to finalize the replacement of mbox with a file per mail (maildir like) storage format (essentially multiple EML files). That is like maildir not an implementation of maildir.

Having said all of that, I opened on of my MBOX files in my profile at random in notepad++ and is started the file with From - Sat May 16 07:32:13 2015 X-Mozilla-Status: 0000 X-Mozilla-Status2: 00000000

So I think your issue probably has something to do with you being on Linux as I am seeing on windows a date format consistent with the C asctime() output https://www.ibm.com/docs/en/i/7.3?topic=functions-asctime-convert-time-character-string

Thanks a lot for your reply and for clarifying the scope of the RFCs! Indeed, I am working on a Linux PC.

So, is there anything I can do in order to maintain compatibility with pine or mutt? It seems that TB vs. pine/mutt simply disagree on the format, with nothing to be done; use either one or the other but not both? (The time format used still seems to have changed with TB 91).

Best regards, David

None of the applications are meant to be interoperable. I really have no idea what file format, or sub format of mbox they use, but one topic I looked at suggested pine on windows does not use a format recognizable to any other application.

In a world of GUI operating systems, who is really going to use a command line text based email client like mutt or pine. My understanding is pine does not even support unicode. but either way I suggest you accept they are not data compatible and maintain separate data stores for you various mail clients.