Digests, ie collections of messages, come in three general flavors:
1. Those which follow the official rfc digest structure. These are true digests and can be parsed unambiguously back into their component messages.
2. Those which don't follow the official structure but which have simple, standard separators between messages (a line of dashes or a line of equal signs). Using a heuristic, Eumorpha can do a reasonably good job with these. But it's important to note that this format is a pseudo-digest, not a true one, and carries with it the possibility of ambiguity, and if that happens, there's nothing Eumorpha can do to get around it.
3. Those which have no separators at all between messages. These are the trickiest to convert, and have the most potential for ambiguity. At the moment, Eumorpha makes no attempt to handle these (you can use MailConverter, however).
THERE IS NO WAY IN GENERAL FOR MailConverter TO KNOW WHICH CASE IT'S DEALING WITH! To minimize ambiguity, it depends on some help from the user.
Eumorpha proceeds as follows. It looks at the first line, and if it's a mail header, the file is assumed to be type (2). If the first line is not a header, it will look through the entire file for some indication that the file is a type (1).
The reason MailConverter considers type (1) last is that true digests will be converted more or less correctly by the type (2) converter, but not necessarily vice versa; and because type (1) files, despite being the official standard digest format, don't occur all that often in the real world.
On the other hand, type (1) is clearly preferable when appropriate, since it's unambiguous. So if you have a file of type (1), it's a good idea to force type (1) conversion by ensuring that the first line is not a mail header.
The question that remains is, how can type (1) files be recognized by the user? This is somewhat complicated to explain in general (see rfc934 for specifics), but typically they will contain a line that looks something like this:
------- start of digest (x messages) (RFC 934 encapsulation) -------
