Enh: provide alternative export format(s)

The mbox format is widely used and convenient but there are several variations in how From_ lines are handled. This can result in corrupted messages on import.

It would be useful to provide an export format that avoids these problems.

The issue does not identify any files, tests, or entry points to inspect. First determine which export code handles mbox output and clarify the alternative format and From_ handling requirements before implementation; done means providing an export format that avoids the described import corruption.

Written by the indexing model from the issue text.